Swarm. The swarm can occupy another creature's space and vice versa, and the swarm can move through any opening large enough for a Small snake. The swarm can regain hit points or gain temporary hit points.

Eternal Mind. if Zy'ren is destroyed then within 5d10 days he will possess a swarm of vermin and come back to life

Bite. Melee Weapon Attack: Targets make a dc 10 athletics saving throw they take 1d10 piercing damage on a success and twice as much and the condition poisoned on a fail

Spellcasting.  Zy'ren is a 8th level wizard and can cast any spell from the wizard's spell list as well as Zy'ren's Excruciating Thunderbolt

Mind Breaker  Zy'renncan, every 2 turns choose to attempt to turn one nearby creature into a undead under his control. The target must succeed on a dc 10+X int saving throw, where X is the targets current HP subtracted from the targets total HP