Cranium Rat Colony

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

Swarm. The colony can occupy another creature’s space and vice versa, and the swarm can move through any opening large enough for a Tiny rat. The swarm can’t regain hit points or gain temporary hit points.

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

Hive Mind. The Cranium Rat Colony has a collective mind that grows stronger the more cranium rats there are nearby. Every hit point represents a rat in the colony. When the colony falls below 100 hit points its mental stats (Int, Wis, Cha) and access to spells starts to diminish according to the following chart. In addition, when below 200hp its truesight becomes darkvision instead: 

HP        Int     Wis     Cha     Spell DC     Spells
1-49       7       14         7            10            command, comprehend languages, 
50-74   12       17       12            13            confusion, detect thoughts, mind spike, spider climb
75-99   17       19       17            15            mass suggestion, synaptic static
100+    22       22       22            18            mental prison, dominate monster, power word pain

Multiattack. The colony casts one spell, then makes two bite attacks. Alternatively, the colony can cast a 2/day spell and then mind spike. When below 100hp, the colony only makes its two bite attacks, or cast a single spell.

Bites. Melee Weapon Attack: +6 to hit, reach 0 ft., one target in the swarm’s space. Hit: 14 (4d6) piercing damage, or 7 (2d6) piercing damage if the swarm has half of its hit points or fewer, plus 22 (5d8) psychic damage.

Spellcasting (Psionics). As long as it has more than half of its hit points remaining, the swarm casts one of the following spells, requiring no spell components and using Intelligence as the spellcasting ability (spell save DC 18):

At will: command, comprehend languages, detect thoughts, mind spike, spider climb

2/day each: confusion, synaptic static, mass suggestion, mental prison, dominate monster, power word pain

Illumination. The swarm sheds dim light from its brains in a 5-foot radius, increases 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 extinguishes the light.

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

Attack. The colony makes one bite attack

Cast Mind Spike. The colony casts Mind Spike

(Costs 3 Actions) Mind Blast. Creatures of the colony's choice within 60 feet of it must succeed on an Intelligence saving throw (against current spell DC) or take 32 (5d10 + 6) psychic damage and be stunned for 1 minute. A target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.