Yek

Devouring Swarm. If a creature has three or more yek attached to it from a bite attack at the end of its turn, the creature must succeed on a DC 12 Constitution saving throw or its Constitution score is reduced by 1d4 as the demons feast upon the creature's flesh.

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

Pack Tactics. The yek has advantage on attack rolls against a creature if at least one of the yek's allies is within 5 feet of the creature and the ally isn't incapacitated.

Multiattack. The yek makes one bite attack and one claw attack.

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 13 (3d6 + 3) piercing damage, and, if the target is Large or smaller, the yek demon attaches to it. While attached, the yek demon can make this attack only against the target and has advantage on the attack roll. The yek demon can detach itself by spending 5 feet of its movement. A creature, including the target, can take its action to detach the yek demon by succeeding on a DC 13 Strength check.

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 10 (3d4 + 3) slashing damage.