Beholder

OG Beholder
As a child, I always wondered how the hell any party would ever kill a beholder when you'd be subject to like 1d4 save or pretty much die effects per round. (Child me had relatively little imagination when it came to tactics and strategies). Nowadays I'm actually quite keen on using such a creature as a dragon-alternate. Rather than pinning it down and taking cover as one does with dragons, I imagine fighting a beholder would be like ambushing a tank. You need to make sure the big gun isn't pointed at you, you can't rely on cover more than once on account of disintegration, and you need some manner of dealing with random debuffs. Someone was complaining about all the RNG rolling one has to do in determining what eyeballs are available in what angle of attack round to round but honestly that sounds like half the fun of making this lone 'boss monster' really stand out from other fights.

That said Beholders are popular but I have never used one, so that's all I have to say here.

Sunset Realm Beholder
 I have two takes on the Beholder. The 'default' type is an artificial life form designed by either serpents, or by alves emulating the dread weaponry of the fallen Serpent Empire. Either way, they are grown from the heads of wizards, and as such Beholders tend to resemble huge mutant heads of the source head. Despite the resemblance, however, there are only spell-souls inside the creatures, and they are no different

1. Serpent Beholder- Scaly and befanged, slit-pupiled and horrid. Able to unhinge the jaw to swallow things whole.
2. Alf Beholder- Elegantly arched eyestalks, skintones reminiscent of fashionable cosmetics from a thousand years ago, an imperiously sneering nightmare vision of an Alf, beauty and mutation magnified to ten times the size of a regular head.
3. Ningen Beholder- Massive and piscine, a giant anglerfish with 10 lures and needle teeth, skin black and bioluminescent in spots. These gilled grotesqueries are found in the depths of the sea and are known as 'Eyes of the Deep.'
4. Great Orb of Eyes- Inspired by the BFRPG's reskinned (and much simplified) version of the beholder, this is just a cluster of huge eyeballs hosting spellwisps, the work of master sorcerers who found the giant head version more disturbing than a bunch of eyeballs.
5. Moon Eater ( this is the second variety see entry below)

 Their eyeballs host spell-wisps of terrible power, and in the absence of their masters, Beholders idly guard ancient ruins and hoard other spellwisps with which to communicate and breed. Fortunately, this merely results in hybrid spellwisps being bred, and at worst infesting a spell-scroll or something, rather than entire new beholders being spawned. They are colonial beings, a collection of spells piloting a meat-vessel that they are bound to until it is slain, at which point the spellwisps go their separate ways. The wisps are likely sick of hundreds of years of servitude, and it is unlikely that a wizard could learn a spell from a dead beholder's component wisps, though an intact eye-stalk might be good for just one more spell.

Beholder Spellwisp-Eyes

1. Alvish Bewitchment- As Charm Person. Rather than simple light-bending illusion, this spell creates hallucinations within the brain of an individual that allows perception to be utterly controlled. As with all mental effects, the soul itself cannot be controlled, only convinced or coerced, but the perceptions of a living body can be fooled, and the soul with it. The illusions are tailored to things with humanish psychology and are ineffective against beasts, and undead see with their souls rather than their body and are similarly immune.
2. Alvish Override- As Charm Monster. Many monsters (and people) were artificially created by tinkering with the artificial 3rd Sun, the Alf Star, the Sorcerer's Sun, and have failsafe codes, instincts of obedience wired into their blood. This spellwisp essentially just runs through a list of access codes, attempting to brute force guess the password that will hack the physical wetware of the brain, leaving the soul to curse impotently within a rebellious body. Making one saving throw is no guarantee that a being has no genetic curse-coding lurking within- it merely means the litany of codes did not include the correct code in time before the spellwisp grew tired of guessing.
3. Sleep-  In the event no subtler control can be gained, mortal brains can oft be disabled and forced into rest mode by even weak spellwisps. Souls of sufficient willpower can rouse their bodies from this false slumber with no effort at all, of course. Elvish beholders utilize Slumber, a variant that has no HD restriction, but is single-target.
4. Telekinesis, 250 pounds. While heavily armored warriors are too heavy to be moved by the spellwisp that effects this telekinesis, people of slighter build and smaller packs can be moved into peril with no saving throw (though acceleration to high speeds takes a great deal of time). Large rocks may be lifted as deadfalls as well, and the utility of this eye, once used for simple manual labor, should not be understated
5. Petrification Ray- A short range (7 feet) basilisk soul ray, spellwispified (see basilisks for details there). The limited range is a failsafe to prevent the Beholder from petrifying itself by shooting a mirror.
6. Disintegrate Ray- perhaps the ultimate weapon of a beholder, this highly destructive beam of negative sunlight, kept barely in check by a specialized spellwisp from the mirror realm, undoes the elemental covenant, blasting matter into shadow and nothingness. It has a range of merely 5 feet, so even a spear is sufficient to stay out of danger.
7. Fear- A bit of a gimmick, this spellwisp merely tickles the adrenal glands, triggering a fight-or-flight response from most living beings. One might wonder why, when Charm effects are available, such an effect would be included, but the answer is simple. Alves sometimes wished to be free of the presence of people they disliked, but could not kill them without risking a faux pas. Charm effects are useless when attempting to get someone to leave you alone, and so this effect was included in beholders to smooth over social incompatibility.
8. Slow- As Beholders are not particularly fast, this decently-ranged effect allows them to close to melee range and utilize their more powerful effects, preferably on targets who are already Feared.
9. Heretic's Lance- As Cause Serious Wounds. this spellwisp is typically a imp or cherub in service to some forgotten and spiteful god (Isfrix the Demon Lord of Hate, probably). With a range of 12 feet, this effect, along with the Death Ray, is used against targets who prove resilient to the Death Ray and refuse to come within range of the other rays.
10. Death Ray- With a range of 10 feet, the spell wisp used for this effect is classically a rogue psychopomp, empowered to pluck souls from their bodies. The effects are all-or nothing and, against many targets, the Heretic's Lance is actually the more reliable option.
11. Spellwisp Suppression- as anti-magic cone. While most spellwisps are composed of light or shadow, the central eye of the beholder contains a mote of Darkness. Spellwisps that it regards immediately flee from this realm of existence, fearful  of obliteration, and as such, spells cast in the direction of the eye fail, magic items cease their functioning, and a jolly good defense is had by the beholder. The beholders own wisps are not exempt, and while the central eye is open, the beholder may not fire spell-rays in that direction

There is another creature similar in function, though metaphysically alien.

Moon Eaters
These tentacled creatures that inspired the creation of Beholders are among the boldest of darkspawn known, for they nibble on the eldritch moons that bedevil the moonlands, and carry around chunks of moon-stuff that still glow with the effects of the light of the moon, feeding upon the stolen light. Once equipped to their satisfaction with moon-stuff, they may descend from the high places of howling darkness between the stars to try their luck at stealing sunlight. Not from the sun itself, of course, oh no, they would not dare... but the sunspawned mortal races and their magical baubles are irresistable to Moon Eaters.

A Moon Eater is a squiggly spheroid mass of midnight black, ontologically vague tentacles that clutch glowing moon-fragments and other baubles. They are devoid of set features, possessing an amorphous form, and do not have a predictable set of effects, as they simply brandish moonstones, stolen fairies, spell-wisps half-trapped in magical scrolls or items and pressed into servitude, haunted skulls, etc etc... an example mirroring Beholders is shown below, but there are endless possibilities of spell and moon combos.

1. Nightmare Anchor- A tormented human soul trapped in a nightmare of its own making, calling out for assistance, and trapping those who heed its call. Those drawn into the nightmare leave their bodies uninhabited and watched over only by the shadow, which is naturally more inclined to listen to the Moon Eater's Darkspawn will than the light-souled human friends of the absent soul. unless they can save the trapped soul and return.
2. Animal Dream- An idyllic dream-afterlife of prey animals, channeled through the skull of a longdead animal. Other prey animals may be tempted to enter the paradise, and predators too, though for different reasons. As with the Nightmare Anchor, once the body is vacated in favor of another realm, it is ruled by the shadow and prone to allying with Darkspawn immediately.
3. Captured Slumbermaid- A fairified spellwisp of a sleep spell, pinned inside a dark tube-tentacle and squeezed to encourage cooperation with the Moon Eater's wishes. The fairy would rebel even under pain of death if an Alf was being threatened, but so long as the Moon Eater preys on humans, the relationship is more symbiotic than you might initially assume.
4. Gravity Elemental- A form of earth elemental found at the very edge of the deep darkness of the earth, now corrupted to serve the Moon Eater instead of the laws of the world. As it is a very small elemental, it may only change the subjective gravity of objects of up to a large man's weight.
5. Winter Moon Chunk- Those exposed to the narrow beam of light freeze solid (save to dodge). Nearly any amount of liquid can be frozen. Thawing out frozen people is dangerous unless a warm bath of holy sun-warmed water is used.
6. Slurping Tentacle- as the Moon Eater is a darkspawn, anything it devours (and it can devour anything) will have the light and law leeched from it until nothing but inchoate matter (and presumably a wailing spirit) remains.
7. Fear- Honestly, the writhing of a darkspawn is naturally fearsome to any sunlit mortal beings, and a tentacle that lunges at peoples faces, unfolds like a tooth-filled umbrella, and screeches like a dying trumpet is enough to spook most things away.
8. Air Rebellion- One tentacle whispers like a mouth rather than clinging to a chunk of magic rock. The air listens and becomes like cold gelatin, slowing movement around an unfortunate being. Air, Earth, and Water- all are more similar than you might think when nostalgic for their primordial form of undifferentiated dark matter.
9. Dark Jab- As Cause Serious Wounds. It looks like a simple jab of black tentacle impaling the target, but darkspawn have no set composition. It can harm anything, because the tentacle is potentially everything harmful all at once.
10. Soul Yank- While the Dark Jab affects the material, a Darkspawn is not exactly a physical being, and can drag a soul from a body with no more fuss than you might pull someone out of a sweater.
11. Eclipsing Regard- identical to the dark-spellwisp beholders have, smaller beings of light cannot stand before a being of such deep shadow, and the eyeless 'gaze' eclipses the small sunlit laws spellwisps bring with them in favor of the quiet stillness of the dark. Stronger lights like souls and moons and suns cannot be eclipsed.