in “Dawn of the Overmind“, below the Undercity of Penumbra, deeper than the Tuennelscape is the Nethermost. This is where the Annulus lies guarded by the Unredeemed Beast. But before that, a gargantuan swarm of illithocyte need to be dealt with – which my level 19 party did by using chain lightning to wipe them out – a total of 360 points of damage (double vulnerability). So my PCs never really interacted with them but here they are.

Swarm of Illithocyte
Medium swarm of Tiny aberration, neutral
Armor Class 12
Hit Points 36 (8d8)
Speed 30 ft.

9 (-1) 14 (+2) 10 (+0) 4 (-2) 11 (+0) 14 (+2)

Damage Resistances acid, poison; bludgeoning, piercing, slashing
Damage Vulnerabilities lightning
Condition Immunities Charmed, Frightened, Grappled, Paralyzed, Petrified, Prone, Restrained, Stunned
Senses darkvision 30 ft., passive Perception 10
Languages telepathy 30 ft.
Challenge 5 (1,800 XP)

Creature Sense. The swarm is aware of the presence of creatures within 100 feet of it that have an Intelligence score of 4 or higher. It knows the distance and direction to each creature, as well as each creature’s Intelligence score, but can’t sense anything else about it. A creature protected by a mind blank spell, a nondetection spell, or similar magic can’t be perceived in this manner.

Magic Resistance. The swarm has advantage on saving throws against spells and other magical effects.

Psionic Digestion. The swarm feed off mental radiation unique to Penumbra and do not require food or water.

Swarm 2.0. The swarm can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny aberration. The swarm can’t regain hit points or gain temporary hit points. A swarm takes half damage from attacks that
use an attack roll and double damage from effects that require it to make a saving roll. Creatures that are not swarms are impaired while they remain in the swarm’s space or within 5 feet of it and have disadvantage on attack rolls and Constitution saving throws to maintain concentration. The area of the swarm is considered difficult terrain.

Slithering Doom. The swarm attacks all creatures that share a space with it and automatically do 14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer.

A creature that suffers piercing damage from swarmfighitng must make a DC 15 Constitution saving throw otherwise the area of the bite transforms into a brief mist of crystalline fragments after one round. At this time, the victim suffers 5 (1d10) psychic damage. If the swarm is still sharing the same area, it can inhale some of the fragments and draw energy from them, adding 1 point to its hp for the next 1 minute. This is the only way that the swarm can heal or gain temporary hps.