Rapid Shapechanger. The yochlol elder can use a bonus action to polymorph into a form that resembles a female drow or giant spider, or back into its true form. Its statistics are the same in each form. Any equipment it is wearing or carrying isn’t transformed. It reverts to its true form if it dies.

Maddening Webs. A creature that starts its turn in a space filled with webbing conjured by the yochlol elder's web spell takes 7 (2d6) psychic damage.

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

Spider Climb. The yochlol can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.

Innate Spellcasting. The yochlol’s spellcasting ability is Charisma (spell save DC 16). The yochlol can innately cast the following spells, requiring no material components:

At will: detect thoughts, web
3/day each: stone shape, telekinesis
1/day: dominate person

Web Walker. The yochlol ignores movement restrictions caused by webbing.

Multiattack. The yochlol elder makes two melee attacks.

Slam (Bite in Spider Form). Melee Weapon Attack: +9 to hit, reach 5 ft. (15 ft. in demon form), one target. Hit: 8 (1d8 + 4) bludgeoning (piercing in spider form) damage plus 28 (8d6) poison damage.

Amorphous Flurry (Demon Form only) (Recharge 5–6). The yochlol elder makes four slam attacks.

Noxious Mist Form. The yochlol elder 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 elder 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 take 10 (3d6) poison damage and be poisoned until the start of its next turn. While poisoned in this way, the target is incapacitated.

Teleport. The yochlol elder teleports, along with any equipment it is wearing or carrying, up to 60 feet to an unoccupied space it can see.