Crazy Square Swarm

Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Tiny insect. 

Might of the Master. The following numbers increase by 1 when your proficiency bonus increases by 1: the defender’s skill and saving throw bonuses (above), the bonuses to hit and damage of its bite attack.

Requires Swarmkeeper Bonus Action

Bite. Melee Weapon Attack: Swarmbaker WIS modifier + proficiency to hit, reach 0 ft., one target in the swarm's space. Hit: 1d4 + proficiency piercing damage for every 10 Squares in the swarm, rounded down.

Every time this attack is used, roll a Crazy Cannibal Surge (1d20). The DC begins at 1 and increases by 1 for every success until a fail. On a fail, roll 1d20: that many squares die due to cannibalism.

Protect the Queen. Beginning at level 10, the swarm can use its reaction to impose a deficit on rolls to hit when they are in a creature's space.