Avatar of Lolth

Empowered Web. The Avatar of Lolth’s web spell fills a 30-foot cube and deals 10 (3d6) poison damage to any creature restrained in it at the start of their turn. The webbing is immune to all damage except magical fire. A 5-foot cube of the web is destroyed if it takes at least 20 fire damage from a spell or other magical source on a single turn.

Innate Spellcasting. The Avatar of Lolth’s spellcasting ability is Wisdom (spell save DC 25, +17 to hit with spell attacks). She can innately cast the following spells, requiring no material components:

At will: command, counterspell, dancing lights, darkness, detect magic, detect thoughts, dispel magic, faerie fire, invisibility, levitate (self only), ray of sickness (3rd level), spirit guardians, web (empowered version)
3/day each: dominate person, harm, heal, insect plague, power word pain
1/day each: power word kill, time stop

Legendary Resistance (3/day). If the Avatar of Lolth fails a saving throw, she can choose to succeed instead.

Magic Resistance. The Avatar of Lolth has advantage on saving throws against spells and other magical effects.

Shapechanger. The Avatar of Lolth can use its action to polymorph into a form that resembles a medium humanoid (usually a female drow), a huge spider, or back to her true form, which is a huge humanoid-spider hybrid. Her statistics, other than her size, are the same in each form. Any equipment she is wearing transforms with her, but nothing she is carrying does.

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

Web Walker. The Avatar of Lolth ignores movement restrictions caused by webbing.

Multiattack. In spider form, the Avatar of Lolth uses bite and makes three attacks with flying spiders. In humanoid form she makes three attacks with flying spiders and uses an at-will spell. In hybrid form, she can attack like a spider or a humanoid.

Bite (Spider or Hybrid Form Only).  Melee Weapon Attack: +14 to hit, reach 5 ft., one creature. Hit: 20 (3d8 + 7) piercing damage, and the target must make a DC 21 Constitution saving throw, taking 32 (5d12) poison damage on a failed save, or half as much damage on a successful one. If the saving throw fails by 5 or more, the creature is poisoned for 1d4 rounds, and is paralyzed while poisoned in this way. The creature can repeat the saving throw at the end of each of its turns, ending the effect on a success. Otherwise, it drops to 0 hit points and makes death saving throws with disadvantage.

Flying Spiders. Ranged Weapon Attack: +14 to hit, range 60 ft., one target. Hit: 9 (1d4 + 7) piercing damage plus 10 (3d6) poison damage.

Teleport. The Avatar of Lolth magically teleports, along with any equipment she is wearing or carrying, up to 120 feet to an unoccupied space she can see.

The Avatar of Lolth 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. The Avatar of Lolth regains spent legendary actions at the start of her turn.

At-Will Spell. The Avatar of Lolth casts one of her at-will spells. 

Teleport. The Avatar of Lolth uses her Teleport action.

Summon Yochlol (Costs 3 Actions). The Avatar of Lolth magically summons a Yochlol who appears in an occupied space within 60 feet of her. The summoned Yochlol acts as an ally of its summoner and can’t summon other demons. It remains for 10 minutes, until it or its summoner dies, or until its summoner dismisses it as an action.