Innate Spellcasting. Zaithar's spellcasting ability is Charisma (spell save DC 18, +9 to hit with spell attacks). It can innately cast the following spells, requiring no material components:

At will: arms of Hadar, cause fear, darkness, mind sliver

3/day each: bestow curse, fear, hunger of Hadar

1/day each: eyebite, maddening darkness

Legendary Resistance (3/Day). If Zaithar fails a saving throw, it can choose to succeed instead.

Dark Regeneration. Zaithar regains 20 hit points if it has at least one hit point and isn't in an area of bright light. If Zaithar takes radiant damage, this trait doesn't function at the start of Zaithar's next turn. Zaithar is only destroyed if it starts its turn with 0 hit points and doesn't regenerate.

Incorporeal Movement. Zaithar can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Shadowed Displacement. Attacks against Zaithar have disadvantage while the demon is in an area of darkness.

Shadow Stealth. While in dim light or darkness, Zaithar can take the Hide action as a bonus action.

Multiattack. Zaithar makes two attacks with its claws.

Claws. Melee Weapon Attack: +11 to hit, reach 5 ft., one creature. Hit: 18 (3d6 + 6) psychic damage or, if Zaithar had advantage on the attack roll, 30 (6d6 + 6) psychic damage.

Shadow Blind. Zaithar casts the Blindness component of the blindness/deafness spell on a creature within range.

Fade Dodge. Zaithar forces a creature to reroll one attack roll against it. Zaithar must not be in an area of bright light to use this reaction.

Zaithar 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. Zaithar regains spent legendary actions at the start of its turn.

Move. Zaithar moves up to its speed without provoking opportunity attacks.

Claws. Zaithar makes an attack with its claws.

Shadow Blind (Costs 2 Actions). Zaithar uses its Shadow Blind action on a creature within range.