1d20 Unrealistic but Gameable Insanities

There are occasional 'insanity' effects in D&D, but as one of my players has said, trying to model actual mental illnesses in a roleplaying game mechanically can come off as all manner of insulting, ignorant, insensitive, and so on (as one example of many, the AD&D tables use such outdated terminology that the name of the condition and what is actually described do not even match).

My approach to the matter is to have insanities be gonzo mental mutations/curses that are primarily focused on making the game more interesting rather than modeling anything from real life. The most common means of obtaining an insanity is from learning forbidden knowledge where the insanity is the price you pay for the knowledge/abilities gleaned, and if the insanity is removed, so too is the knowledge lost. This allows for roleplaying the insanity to the specifics- one example would be from my own campaign, where players had access to a scroll that showed those who read it flashes of futures, one of which was the fall of the human world to brain wasps in a distant future if their current sorceress-princess was driven out of the society. Upon rolling 'Homicidal Mania,' Ser Arsem became convinced he needed to murder certain people to prevent this terrible future from occurring, and it was a pretty useful plot point that led to a lot of interesting misadventures. It was up for debate if he was acting on prophecy or insanity. I think leaving the precise details to the players is the best thing to do, so it becomes an aspect of the character rather than random 'ok you are NPC now and don't get to play' thing.

1. Chaotic- You cannot take the same action twice in a row, be it during combat or downtime. At the GM's option, repeating plan may be impossible as well if any pattern of behavior is being established. Any attempt to predict your actions (such as reading your mind to see what you are thinking of doing next, or time-travel) will be bamboozled.
   
2. Elvishness- Due to ruinous nostalgia of past lives, excessive interest in the aesthetics of a scenario, a sudden desire to recite philosophical poetry or create art, or talking to spell-wisps, ghosts, bugs, etc, you have a 1/20 chance of losing touch with the urgency of any crisis situation and instead focusing on something whimsical instead. If you create something artistic to share with the group from this whimsy, you are eligible for a small bonus of XP, along the lines of an X from Die Trying at GM discretion.
   
3. Hypersanity- What if it's not you, but everyone else? Once per session, upon encountering some common mundanity, like 'sitting in chairs to eat' or 'shaking hands,' declare that an act of insanity and refuse to partake in it henceforth, regardless of social consequences. If your list becomes unmanageably long do not fret about what you forget now and then, just try to make up for it upon remembering (like throwing away all the chairs to prevent a similar slip-up).
4. Parallaxis- Your mind, attuned to realities congruent and less so, cannot help but provide you with extra information about these parallel worlds. You do not believe yourself to be lying, but everything you say is embellished with things that you forget are not locally true, which makes it difficult for people to believe even the true parts when you mention a town that was never founded being vital for this month's crop supply. These false ideas are not entirely useless- to use the prior example, the unfounded crop-town may be a valley of rich soil unclaimed by anyone else.
5. True Love- Pick a character, or perhaps even an idea (It need not be romantic love). You would do anything for them, and if they perish, it might as well be the end of the world for you. You are immune to any effect that would turn you against them. 
6. Jekyllism- You have an alternate self, the shadow, the id, maybe a cannibalized twin, whatever you call it, and you can call it forth, and it you, at will, though altered mental states sometimes cause an involuntary swap (such as a Fear effect causing one personality to retreat, calling forth the other). It is your opposite, or near to it, and your stats are reversed and even your class is different (Fighter-MU and Cleric-Thief being the default 'swaps').
   
   The other personality will seek to avoid this 'insanity' being removed, or even seek to remove you and become the only self, but is not unreasonable and acts in their own best interests and will cooperate on important matters.
   
   Depending on context, the other self may not be a facet of yourself, but a dream, or a fictional character, or a ghost or something, in which case they are not so much 'opposite' as 'just another person' and may have knowledge completely unknown to you.
   
7. Homicidal Mania- Upon meeting new people (having them talk to you in enough depth to learn each others names is the rough trigger here) you have a 1-in-20 chance to decide they need to die. You may scheme for subtle assassination plans rather than defaulting to instant stabbing, but if delayed for too long you will lose the ability to gain XP, and eventually the ability to sleep.
8. Nightmare Host- Whenever you sleep, you become a Nightmare Incursion, a dungeon of sorts that can be entered and altered from within, and is host to monsters symbolic of yourself. This effect occurs and is permanent upon death, and those slain within/killed by the monsters of your nightmares become nightmare incursions themselves. The wandering encounter rates are such that if you are asleep, no one near you will be for long.
   
   As it may soon be relevant, sleepless nights reduce all stats by 1 and prevent any healing, recovery of spells and abilities, and so on.
9. Determined- Pick a goal you have- you are obsessed with its completion and cannot take downtime activities in the service of anything else, and should justify most actions within this context, or grumble mightily when doing unrelated things. If you are slain before it is completed, you immediately become undead and continue trying to accomplish this goal devoid of the weaknesses of the living (as a PC or NPC depending on preferences). You gain bonus XP for following your goal, however, and pick a new goal if you complete one.
   
10. MegaMasochism- You register pain as pleasure, and gain 1d6 temporary HP whenever struck for damage. This temporary HP is lost at the end of a combat or otherwise defined scene, as it is a brief high and your wounds are still wounds. You also lose any dodge-based AC bonuses, and show up with 1d6 less HP after any instance of downtime due to self (or otherwise) flagellation.
    Previously pleasurable activities lose their luster and so things like food, drugs, soft beds, and even healing effects (unless the healing process is painful/does not numb pain) provide only half their previous benefits, rounding down.
    
    Naturally, any pain-based attempts at coercion are entirely ineffective.
    
11. Awakened Sleeper- While asleep, you function as though mostly awake, though your speech is unintelligible and you must simultaneously contend with the dream realm as well as the waking world, (assuming the GM is too busy to run both realms at once, apply a 50% chance to act as Confused each action taken). While useful for night-time ambushes, sleep spells, and so on, this is largely an annoyance and it may be easiest to bind yourself upon rest so you do not wander.
    
12. Darktongue- You constantly mutter and babble, though less so if you occupy your mouth with a pipe, or food, or a hand over it. Fragments of out-of-character chatter make their way into diegetic conversation, impairing auditory stealth and conversation, and your muttering may disturb others- if something especially odd is said, it may reroll reaction or morale of friends and enemies alike. Spellcasting is most direly affected- when you cast a spell, it is subject to wild magic, temporary spell mutations, etc etc
    
13. P-Zombie- You lose your soul and your mind, have no internal experience of being, and are immune to any such effects reliant on you being more than a chemo-electrical meat-puppet. However, the empty thing that I will refer to as 'you' for sake of conversation behaves as though 'you' still had consciousness, and acts as though 'you' were affected by such effects. Mind-reading or 'detect' spells targeting you will yield no results, however, as there is 'nothing there.' Philosophers must save vs nervous sweat in your presence.
14. Fight/Flight- When put into crisis situations/threatened with physical harm, you must make a Will save (or whatever save is most appropriate) or you must either enter a berserker rage in which you deal and take maximum melee damage and must attack(or at least take violent offensive action like throwing Fireballs or chasing your foes bellowing) until all threats are eliminated, or you may flee as fast as possible until you have evaded, hidden from, and otherwise lost all pursuing threats and given yourself a good 10 minutes/1 dungeon turn to calm down.
15. Amnesia- You forget the specifics of your past- you know what a human is, but you don't know WHO a human is. You know what spaghetti is, but not if you like it. All that remains is a dim recollection of 'ah yes, I was a human*, I did, human things.' The memories are retained within the soul or the brain (depending on lore) but cannot be accessed until the seal upon them is removed. Some abilities and skills may be lost, others may remain via muscle memory and unconscious influence.
    
    *Obviously replace human with dog if you are a dog or something 
16. Hyperbolic Optimism/Pessimism- Upon waking, you have a 50% chance to be either rabidly optimistic or nihilistically pessimistic for the rest of the day. While initially amusing it soon begins to wear on those exposed, and each subsequent day of the same ludicrous cheer or crushing gloom reduces the morale of retainers by 1 until the swap is made back for a change of pace.
    
    For downtime, you may take two downtime actions, but each has double chance of mishaps occurring due to hasty decisions and self-sabotage.
    
17. Blabbering Honesty- You cannot lie, and if someone is lying and you think you know the truth, save or blurt it out.
    If you are an Elf or similar fey critter, you instead lose all your magic powers if you lie overtly (though lies by omission are fine).
18. Nightmare Curse- You are not quite possessed, but infested by a nightmare creature that manifests as aberrant behavior. Roll your random curse on page 236
19. Contrarianism- When told to do something, save or attempt to do the opposite, or in the case of an exact opposite being untenable, something counterproductive to the intended order. Your own intended actions can trigger this (ie, jumping into a pit instead of over it), but only if you have gone a session without causing some zany misadventure and are now jonesing for contrarian wackiness.
    
20. Darkmind- Exposure to forbidden elements has altered your mind-concept in alien ways incomprehensible to mere diegetic beings. Your character is now the character of at least one other player in addition to yourself, providing them with a wealth of new ideas and behaviors.

Giant Gar, Goyle Gar, and Gas Spore

 AD&D Giant Gar
This is just a big 8HD, AC as plate, danger fish that swallows you whole on 20's and has a 5d4 bite.
There are also giant Pike later in the book, but honestly I feel like those entries (and any other oddly specific giant fish) entries could probably have been condensed to a single 'Giant Fish' section, or even been a footnote in the 'Shark' section mentioning 'the shark statblock can also serve for various predatory fish.

AD&D Gargoyles
Gargoyles in AD&D are pretty nasty sorts, being immune to nonmagical weaponry (though their AC is merely as Chain), boasting numerous but weak attacks via claw/claw/bite/horn gore attack routine, and of course flight. They are 'predators of a magical nature' that are 90% likely to attack anything they detect, and are said to enjoy torturing prey to death on account of them being evil monsters. It is not at all clear if they are evilly animated statues intentionally created or some manner of naturally occurring stony monster, but their presence in underground caverns as well as ruins points to the latter, while later modules depicted gargoyles that seem directly inspired by stonework.

They also have, bizarrely, an aquatic variant known as the kopoacinth, which swim with their wings and are found in shallow waters with sea-caves.

Though I prefer a friendlier take on these creatures, I think they are pretty good monsters to serve as minions for larger villains (as the book mentions.) They are immune to normal weaponry, making them good at terrorizing townsfolk, flying away with hostages, and providing a demon-like threat without the grab-bag of magical powers actual demons have.
BFRPG Gargoyles
Largely identical to their origin, this interpretation leans more on them being 'evil rocks' giving them better camouflage/surprise chance when they stand motionless and pretend to be statues, and their predations are confirmed to be cruel in nature, as they need neither food, air, water, or sleep.

DCC Gargoyles
While similar to the BFRPG variant, they have AC as Plate+Shield+3 but only 2HD, and normal weapons still inflict damage, but shatter upon striking the gargoyle 50% of the time. This makes them more prone to being 'seemingly invincible, then suddenly shattered' and a drain on weaponry that better represents them being made of stone but not magically resilient, and all in all I think is a better take than the 4HD gargoyles which are a bit of a slog to get rid of if you actually have the capacity to do so.

AD&D Gas Spore- A nasty sort of 'trick' monster which resembles a beholder, but is in fact a sort of lethal exploding fungus balloon. Killing one causes it to explode in a 6d6 blast in a 20' radius, which even with a save for half is pretty rude as a gotcha moment.

Interestingly, despite being an order of magnitude or three weaker than a beholder, I feel like they can only be used properly AFTER the party has encountered a beholder, for that gives the party a chance to realize something is off (primarily the fact that the main eye of a beholder should be cancelling magic as it gazes upon the party, while a gas spore's 'Eye' is just fungal texture.)

Once the functioning of gas spores has been realized, they could become interesting encounter complications akin to an exploding barrel in DOOM, or terrain hazards in rooms with high bridges, narrow walkways, tight halls, and so on.

One thing I had quite forgotten until rereading the entry is that letting them bob balloon-like up to someone, apparently harmless now you haven't detonated it, is in fact another 'gotcha' moment, where the gas spore will 'shoot tiny rhizomes' into someone, die on the spot (without exploding), and infect the target such that they will die in 24 hours and sprout into 2d4 more gas spores. While a fascinating fungal life-cycle, there's no reason to expect an exploding ball to suddenly function as a hypodermic needle injecting its fungal contents upon touch (or so I imagine the process) and so I'm not too fond of the double trickery of this creature, neither of which is something you would expect.


Sunset Realm Giant Fish
Fish can grow giant just like everything else, but I have no special attachment to Gar, and honestly have plenty of other sea-beasts I'd rather use.

It is a common belief that the uglier a gargoyle is, the more fearsome a guardian it will be

Sunset Realm Gargoyle
Gargoyles come in two varieties- church-defenders carved from the hallowed stone of a temple, their elemental souls given humanoid egos by their new shadows. This is a High Alvish technique adopted first by the 3rd solar age sorcerer-king Sarkomand the Omnipotent, then by the 4th age Church of the Stone Sun. In the absence of either, remaining gargoyles tend to be ancient and somewhat battered, lurking in old ruins and defending them for no good reason anymore, making it not at all impossible to negotiate with them if the appropriate ancient language can be utilized and at least some moral similarities to their fallen cult demonstrated (destroying undead is usually a safe bet, as watching over church graveyards to keep the dead under control was a common task, and recovering lost relics, stained glass windows, and so on that the gargoyles liked to gaze upon is another good bribe.)

The other, more sinister origin of a gargoyle is one who is carved from a (preferably extremely obese) humanoid petrified by the knowledge of Yg transmitted via Basilisk or Medusa, reshaped into an entirely new being by the snakey folk responisble. The original soul is usually cast from the body in this process (unless it hangs on out of consent or stubborn determination), the new soul of a gargoyle being an artificial shadow-soul tweaked to the specifications of the creator. Medusa like gargoyle servants, for they are immune to petrification, and they featured predominantly as soldiers in the Oroboron Civil War in which the Serpent Queen Tinnea (Herself a descendant of Vala) tried to take the city from King Samuel Goffnagoff, only to be ousted in open revolt from basically all the human nobles of the land and replaced by Queen Evalyn Goffnagoff, daughter of the king. This event marked the addition of the goddesses Yg and Lumar to the list of unsanctioned cults of the realm, which stands to this day.

That aside, gargoyles are decidedly an urban creature, (either inhabited or ruined) and the church-defending sort tend to know quite a lot about the goings on of priests, for they have nothing to do but gaze stoically from their perches and gossip as they wait for their time to act. As such, investigations of corruptions sometimes bear unexpected fruits when the ancient gargoyles of an abbey are consulted, and truly unforgivable goings on may even bring down the gargoyles of a building to depose of problems. This is not exclusively limited to religious buildings- some gargoyles were built for military defense and surveillance and do away with spies, thieves, and saboteurs, and the religious leanings of gargoyles is a matter of the secret techniques being passed down among the gods enslaved by the Stone Sun pantheon and therefore used in religious projects far more often than anything else.

Sunset Realm Gas Spore- Fungus is poorly understood by scholars, but it is generally agreed that it is the 'plant life' of darkness, in contrast to sun-loving trees and herbs. And while monstrous plants are not unheard of (the twin moons Spring and Autumn producing such vicious shrubbery without fail), it is no surprise that dark-thriving fungus grows into monstrous forms in the dark.

Gas spores are a form of fungus that grows on rotting flesh and other organic matter not defended by the light of life, and they tend to resemble what they grow from, leading to spores that look like human heads, skulls, bloated roadkill balloons, other mushrooms and so on. The similarity to beholders is more a matter of them being 'a spherical floating something' than true mimicry, but the dim lighting and twitchy reactions of dungeoneering typically works in their favor. They are attracted to air currents and slowly fly about by ejecting spores as propulsion, and they explode when violently struck, or when they touch something that registers as 'alive' or at least 'organic' to their mysterious fungal senses. The concussive burst is sometimes lethal due to throwing those caught in the blast to crack their skulls on rocks and so on, but mostly just knocks people prone with some blood coming from the ears and nose, perhaps attracting nearby monsters (1d6 damage, save vs dragon breath for half, per 1' diameter of the spore, the damage dropping off by 1d6 every 5 feet of distance from ground zero. Spores grow to a maximum diameter of 6' before exploding themselves). Anything spore-covered will infect any dead organic matter it touches until either they clean off the spores, or the spores perish in sunlight.

Infested organic matter (such as the rations of a spore-covered adventurer) will look and taste moldy at first, and over the course of the next 1d6 hours will collapse into rot and sprout into a new gas spore. Corpses will provide enough energy to produce 2d4 gas spores. Infested wood will sprout gas spores as well, but at a slower rate, until burnt. Rotten dungeon timber tends to serve as a sort of 'spawner' of gas spores, ensuring a dungeon will have wandering gas spores until all the infestation points have been destroyed.

The idea behind this variant of gas spore, from GMing perspective, is to have them serve as an explosive menace that also serves as ration and corpse depletion, and built in dungeon re-stocking. The players might turn the gas spore life cycle to their advantage, or fail to take appropriate measures and find themselves drowning in the things as everything in the dungeon falls.

Explodestools- Exploding land-mine toadstools were a favorite of mine, depicted in a dragon magazine article I forget the name/issue of. I think a variant gas spore that grows as a bloated mushroom rather than a floating balloon makes for a good potential change, with the major difference being that Explodestools can thrive aboveground, and of course are stationary hazards.