Amorphous. Yibiyru can move through a space as narrow as 1 inch wide without squeezing. She can also traverse enemy spaces as if they are difficult terrain. 

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

Poison. A creature that touches Yibiyru or hits it with a melee attack while within 5 must make a DC 19 Constitution saving throw or be poisoned. While poisoned in this way, the creature takes 13 (2d12) poison damage at the end of each of their turns. A creature can repeat their saving throw at the start of each of its turns, ending the effect on itself on a success. 

Innate Spellcasting. Yibiyru's spellcasting ability is Charisma (spell save DC 22). She can innately cast the following spells, requiring no material components:

• At will: detect magic, ray of sickness, Tasha's caustic brew

• 3/day each: dispel magic, vitriolic sphere, cloudkill

Multiattack. Yibiyru makes two pseudopod attacks. 

Bile Bomb (Recharge 6). Yibiyru launches a high-velocity ball of explosive bile at a point within 100 feet. Each creature in a 30-foot-radius sphere centered on that point must make a DC 19 Dexterity saving throw. A target takes 58 (9d12) acid damage on a failed save, or half as much damage on a successful one. This bile remains in that area for two turns, dealing 4d10 acid damage when a creature enters the area or starts its turn there.

Yibyiru 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. Yibyiru regains spent legendary actions at the start of its turn.

Pseudopod. Yibyiru makes one pseudopod attack. 

Slide. Yibyiru moves up to her speed without provoking opportunity attacks. 

Cast a spell. (Costs 2 Actions). Yibyiru casts one of her spells.