Actions

Multiattack. The yochlol makes two melee attacks.   Slam (Bite in Spider Form). Melee Weapon Attack: +6 to hit, reach 5 ft. (10 feet in demon form), one target. Hit: 5 (1d6 + 2) bludgeoning (piercing in spider form) damage plus 21 (6d6) poison damage.   Mist Form. The yochlol transforms into toxic mist or reverts to its true form. Any equipment it is wearing or carrying is also transformed. It reverts to its true form if it dies.   While in mist form, the yochlol is incapacitated and can't speak. It has a flying speed of 30 feet, can hover, and can pass through any space that isn't airtight. It has advantage on Strength, Dexterity, and Constitution saving throws, and it is immune to nonmagical damage.   While in mist form, the yochlol can enter a creature's space and stop there. Each time that creature starts its turn with the yochlol in its space, the creature must succeed on a DC 14 Constitution saving throw or be poisoned until the start of its next turn. While poisoned in this way, the target is incapacitated.   Variant: Summon Demon (1/Day)
The demon chooses what to summon and attempts a magical summoning. A yochlol has a 50 percent chance of summoning one yochlol. A summoned demon appears in an unoccupied space within 60 feet of its summoner, acts as an ally of its summoner, and can't summon other demons. It remains for 1 minute, until it or its summoner dies, or until its summoner dismisses it as an action.