We know that this is inside the ruined keep the demon cultists squat in. So I'm turning all those humanoids into humans and relating 'em to the cult. Less emphasis on multiple factions here- this is what some people call a 'fortress' style dungeon, where you gotta worry about calling down a ton of unwanted attention if you get spotted by sentries.
Hobgob+gobs Into= Top Cultist- has d4 lackeys fawning over him. Claim to fame is that he knows how to activate the demon circle. If he's wandering away from his chambers, it's probably to offer a sacrifice at the demon circle. If he's desperate, it might be one of the lackeys, but it's probably just treasure or a captured dog. Maybe he's gotten in over his head and would give it up if the demons could be dealt with, or maybe he's grown addicted to the thrill of playing 'the dark wizard.' He probably knows things about the nasty secrets beneath the castle, but not THAT much. He carries his diary, which has demon summoning rituals.
The bugbear can be the enforcer, named something like "Raebgub," your classic big brutal torturer jailer guy. He bullies the bullyable cultists and appointed himself jailer so he can bully any captured prisoners. He's obviously bigger, tougher, and meaner than the other cultists and has a whip and club. He doesn't care about demons, he just likes being boss, and could probably be convinced to betray the Top Cultist and become Top Bandit instead. He's probably taken a fair bit of treasure from others though, and has the keys to the mysterious iron door in the barracks dungeons.
The choker is a psycho with a thing for strangling people. The demons love him, the cultists fear him. This guy stalks the players and probably won't call for help, because he wants to pick them off one by one. He might even pretend to join the players with the hopes of betraying them later. His whole thing is that he'll never stand and fight, and he has superior knowledge of the area.
Formian workers, halflings, kobolds, and orcs are just cultist grunts with shovels and pickaxes and jobs like latrine digging and guard duty. Their discipline is horrible as they don't really expect anyone to come after them, but it doesn't take a genius to shout a warning and double patrols if the players kill a few and then try coming back a week later for round 2. Depending on the reaction check, they could be into the whole demon thing, or just sorta going along with everyone else. On the other hand, if encountered at night or in a dark area, they will have a 'skull lantern' which is not actually a skull, but a regular bullseye lantern with some skullmoth-grub infested meat inside, which they will use to bedevil intruders, cut off pursuit/escape, and make direct confrontation a guaranteed drain on HP unless the players are going around with their own skullmoth light they're attenuated to.
As for the bats, rats, fiendish rats, batswarms, and ratswarms, lets simplify it into rats. FIENDISH rats. How did they get so fiendish, you ask? Well!
|"Golly I sure hope our child doesn't grow up to draw daemonic bestiality online" -My Parents|
Anyway, demon rats. Their origin really doesn't matter, but they've got something very important going for 'em- fire resistance. Normally when faced with a skittering horde of tiny beasts, your instinct is to pour oil and throw molotovs, right? Well these things is fireproof (mostly) and that ain't gonna cut it. But you know what WILL kill them- Skullmoth light!
So, these demon rats are basically just mean rats, so they aren't going to hunt the players down or wander looking for adventurers to eat. What they do do is guard their nest, and what the imps did is hide the holy blade of the once pious lord of the castle that can hurt them in this demon rat-infested basement somewhere in the ruined keep so the little beasts would keep any nosy adventurers from blundering into it. Heck, maybe the rat lair isn't even in the keep, but in the rich district.
Hints of the sword's existence are in paintings of the lord with him posing with a glowy blade, and probably a statue of him in the courtyard too. But when the players find the lord locked away in his tower, they will find that his skullmoth-animated corpse wears platemail (and is the replacement for the owlbear skeleton) but his scabbard is empty! It's hard for players to pick up on a single hint, so here's more
1- Imps may be seen flying to the hiding location of the sword to make sure it's safe. It's the only thing they know of that can harm them, after all, so they're paranoid someone will get it. You might overhear them talking about it if you sneak up on them.
2- That psycho strangler definitely knows where it is (he knows everything about this area), but good luck getting him to talk. The cultists may have heard or seen something too, and could be bullied, bribed, etc into revealing what they know.
3-Remember the glowing skullmoth grubs in the lords corpse will light up the windows to his tower. That will probably catch someone's interest after they see painting of the glowing sword. They might even backtrack to investigate Skullmoth lairs, since you never know if the light could be from that sword, right?
4-The Sword has a prayer inscribed on it, and reciting the prayer with some true faith behind it activates the glow, and a constant echo of the prayer repeating until it is sheathed. The imps didn't know how to shut it up, so keen-eared characters might hear the prayer if they're nearby.
The prayer takes a full round to recite, and once 'active' the sword glows and prays and can smite the normally weapon-immune imps and will probably come in handy against all sorts of evil monsters, but man, forget stealth and subtlety while you're using it in glowy-prayer mode. It's also the 'Lantern Archon' but turned into a goofy magic item rather than an twerpy mascot character or whatever role lantern archons are meant to serve.
Anyway. There are 4 imps on this floor ripe for slaying with this magic sword, which fulfill both the were-rat, and demon encounters in term of 'dangerous weird thing you can't kill easy and is a tough cookie negotiation wise.' They cooperate, but are disorganized, which they can afford to be since mortal steel can't harm them.
1-The Bad Breath Imp, "Corpselicker"- Imps don't bathe, because the bathing ritual is to ward off disease demons, something imps don't have to worry about. This imp has also taken to eating anything its huge, detachable jaw can fit over, and as such it has become tremendously fat and has 4hd despite still being only like 2 feet tall. The foul smell of this imp's breath and rotten-food covered hands gives disadvantage on making con rolls and rallying minions to confront the stinky beast. Every time it eats its own bodyweight, it gains another HD. Due to demonic constitution, it adds everything it eats to its body at a 1 to 1 ratio. It may vomit to attack, spewing forth a disgusting torrent of plague and random things like bones, pieces of furniture, lanterns, etc. He's eaten the cellar key, and killing him (which releases a veritable volcano of garbage) is the easiest way to retrieve it.
2- The Knife-Throwing Imp "Veindrainer"
It flies, it's immune to your arrows (unless you got like magic arrows or something), and it carries 10 daggers to throw at you from safe distances. Will downgrade to rocks if you steal all its daggers, and upgrade to molotovs if it can get them. It likes blood sacrifices and can be made to buzz off if you slit your hand and pour 1d4 HP into its mouth. It gets one of your abilities whenever this happens though. It may eventually become pretty mighty if the players don't catch on, for it will also try to lick up the blood of everything they kill.
3- Whistling Imp- "Whistler" Is master of the rats and commands them via whistling. This imp likes musical instruments and if it ever acquires a good drum, lute, horn, and flute, it will lead the other imps to create a fiendish metal band that can only be defeated with the power of rock, possibly leading into the ANCIENT GONDWANAN SCREAMLORD arc. This imp is black and white and looks like a member of KISS, and if any player goes beyond the obvious reference and realizes that black+white=two colors=pied=the pied piper, give them a cookie for cleverness.
And yeah, this imp has 1d6+1 rats at all times, and can summon 1d6 more in 1d6 rounds. It can command the demon rats as well, and will do so if the sword is stolen.
4-The Armor Imp- "Mr Sir The Smith" This imp has turned a suit of armor into a deadly piloted armor via clever use of ropes, pulleys, and a support system. This only functions in the abandoned blacksmithy of the keep, and there has to be an imp-sized someone inside the suit operating it, but so long as you don't cut the ropes, it's pretty much invincible to most forms of attack. The armor hangs from a system of pulleys and rails and it's operation is not at all obvious if it is found without its imp pilot. Also the other imps can crawl inside for more powers
Filth Imp adds tetanus to the strikes of the armor and a swarm of skullmoths attracted by the stench
Knife Imp throws knives by peeking through a crack then hucking em, in any direction.
Whistling imp can use rats to supplement the movement systems and make the armor suit fully mobile.
If all imps somehow end up in the armor suit, they'll agree this is definitely the way to go and progressively create bigger and bigger suits until they must be defeated in Voltron-esque combat. The armor imp can be paid gold to work as a regular smith, but can only make armors and weapons that give -1 morale/reaction to your enemies and friends because they all look dark lordesque and spiky.
These imps came from a demon-summoning circle hidden under a rug in one of the servant's quarters. The cult doesn't bother hiding it, but the rug is too tacky to leave anywhere else. People making sacrifices and translating the diary of the Top Cultist can learn to summon demons too.
Finally, we have two oddballs- the Thoqqua and a Shocker lizard. For the Shocker Lizard, let's just go back to those demon rats and say one is from a stormy and ionized hell, and make it a rat that can build up +1d6 lightning damage each turn, which arcs to the nearest source of metal on a 1. It also resists lightning instead of fire.
The Thoqqua is actually something I'm fine with. It's a weird burrowing magma worm that got awoken by shenanigans in the depths, and now it has undermined fort walls and left burrows to lower levels. Every time it shows up it just causes structural damage, deforming the fort and making new entrances to a lower level. We'll call it 'Magma Worm' though, just in case of copyright or something.
This encounter chart is 1d10, but as unique encounters get whittled down, you can just reduce the dice size instead of restocking. Once the cult is weak enough to be displaced by new arrivals, you could refill the keep with something else.
2-Bad Breath Imp
5-Top Cultist & Lackeys
9-d3 Skullmoth (Night), otherwise Namless Cultists w/ Skull Lantern
Incidentally, these might spill out into the abandoned rich district.
And the halfassed map. Remember those paintings and statues that hint at the holy sword! Heck, a questgiver in the nearby settlement wouldn't be out of order. And recall that the worm-tunnels also lead down to the next level, as does the barracks dungeon and the cellar.
This is a cloister btw