Aratnid Swarmlossus

Illumination. As a bonus action, the swarm can shed dim light from its brains in a 5-foot radius, increase the illumination to bright light in a 5- to 20-foot radius (and dim light for an additional number of feet equal to the chosen radius), or extinguish the light.

Innate Spellcasting (Psionics). The swarm’s innate spellcasting ability is Intelligence (spell save DC 13) and is treated as an 11th level caster. As long as it has more than half of its hit points remaining, the swarm can innately cast the following spells, requiring no components:

At will: mind sliver, command, comprehend languages, detect thoughts

3/day each: confusion, dominate monster

Telepathic Shroud. The swarm is immune to any effect that would sense its emotions or read its thoughts, as well as to all divination spells.

Vulnerable Legs. The swarm has eight legs. While it has more than four legs, the swarm is immune to being knocked prone or restrained. Whenever the swarm takes 20 or more damage in a single turn, one of its legs breaks away from the body becoming a Large swarm of cranium rats that is unable to use it's spellcasting feature. Each time a leg is broken after the fourth one, the swarm must succeed on a DC 13 Constitution saving throw or fall prone. Any creature in the swarm space or within 5 feet of it when it falls prone must make a DC 15 Dexterity saving throw, taking 21 (6d6) bludgeoning damage on a failed save, or half as much damage on a successful one. For every 2 legs missing, the swarm's speed is reduced by 10 feet.

Legendary Resistance (3/Day). If the swarm fails a saving throw, it can choose to succeed instead.

Multiattack. The swarm makes one bite attack and then as many smothering stomp attacks as it has legs.

Bite. Melee Weapon Attack: +9 to hit, reach 10 ft., one target. Hit: 16 (2d10 + 5) piercing damage plus 5 (1d10) poison damage.

Smothering Stomp. Melee Weapon Attack: +9 to hit, reach 15 ft., one target. Hit: 15 (2d8 + 5) bludgeoning damage. If the target is a creature, it must succeed on a DC 17 Strength saving throw or be knocked prone. The amount of Smothering Stomps the swarm can make at a target is reduced by two for every size category smaller it is than the swarm.

Chittering Chorus (Recharge 5–6). The swarm shrieks in a 20-foot radius. Each creature in that area must make a DC 16 Wisdom saving throw. On a failed save, the creature takes 33 (6d10) psychic damage, and it is frightened of the swarm, for 1 minute. On a successful save, the creature takes half as much damage with no additional effects. A frightened creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

The swarm can take 3 legendary actions, choosing from the options below. Only one legendary action can be used at a time and only at the end of another creature’s turn. The swarm regains spent legendary actions at the start of its turn.

Psionic Tap. The swarm casts a spell.

Corrupted Mana Moonlight. The swarm releases a column of light in a 40-foot long 5-foot radius cylinder. The target must succeed on a DC 16 Constitution saving throw or suffer 10 (2d10) radiant damage on a failure, or half as much on a success. A shapechanger makes its saving throw with disadvantage. If it fails, it also instantly reverts to its original form.