Xerathrax Arachnoloth, Demon Lord of the Abyssal Web

Legendary Resistance (5/Day): If Xerathrax fails a saving throw, he can choose to succeed instead

Magic Resistance: Xerathrax has advantage on saving throws against spells and other magical effects.

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

Web Walker: Xerathrax ignores movement restrictions caused by webbing.

Multiattack: Xerathrax makes three attacks: two with his venomous bite and one with his shadowy tendrils.

Venomous Bite: Melee Weapon Attack: +18 to hit, reach 15 ft., one target. Hit: 36 (4d10 + 10) piercing damage plus 18 (4d8) poison damage. The target must succeed on a DC 26 Constitution saving throw or become poisoned for 1 minute. The poisoned target is also paralyzed. The target can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.

Shadowy Tendrils: Melee Weapon Attack: +18 to hit, reach 10000 ft., one target. Hit: 32 (4d8 + 10) bludgeoning damage plus 27 (6d8) necrotic damage. If the target is a creature, it must succeed on a DC 26 Strength saving throw or be grappled (escape DC 26). Until this grapple ends, the target is restrained, and Xerathrax can't use his tendrils on another target.

Web (Recharge 5-6): Ranged Weapon Attack: +15 to hit, range 10000 ft., one Gargantuan or smaller creature. Hit: The target is restrained by webbing. As an action, the restrained target can make a DC 26 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 18; hp 50; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).

Attack: Xerathrax makes one bite attack.

Dark Energy Surge: Xerathrax releases a surge of dark energy in a 100000-foot radius centered on itself. Each creature in the area must make a DC 23 Constitution saving throw, taking 56 (16d6) necrotic damage on a failed save, or half as much damage on a successful one.

Summon Demon: Xerathrax summons a demon of challenge rating 19 or lower. The summoned demon appears in an unoccupied space within 10000 feet of Xerathrax, acts as an ally of Xerathrax, and remains for 1 minute, until it or Xerathrax dies, or until Xerathrax dismisses it as a bonus action.

Innate Spellcasting: Xerathrax's innate spellcasting ability is Charisma (spell save DC 26). He can innately cast the following spells, requiring no material components:

• At will: darkness, detect magic, spider climb
• 3/day each: dominate monster, etherealness, greater invisibility, plane shift

Xerathrax 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. Xerathrax regains spent legendary actions at the start of his turn.

Attack: Xerathrax makes one venomous bite or shadowy tendrils attack.

Webbing (Costs 2 Actions): Xerathrax magically shoots sticky webbing from his abdomen. Each creature of Xerathrax's choice in a 5000-foot cone must succeed on a DC 26 Dexterity saving throw or be restrained by webbing. As an action, the restrained target can make a DC 26 Strength check, bursting the webbing on a success. The webbing can also be attacked and destroyed (AC 20; hp 25; vulnerability to fire damage; immunity to bludgeoning, poison, and psychic damage).

Plane Shift (Costs 3 Actions): Xerathrax magically teleports himself and up to three willing creatures he can see within 10000 feet of him to the Abyssal Web or another plane of existence.