Innate Stealth. Zathar can take the Hide action even when he is lightly obscured or in an area of dim light.

Damage Split. When Zathar's hit points drop below 50% of his maximum, he can use his reaction to split damage between himself and another creature within 10 feet of him. The damage is divided equally between Zathar and the other creature. This ability can be used once per short or long rest.

Multiattack. Zathar makes two attacks: one with his Shadow Dagger and one with his Shadow Claws. 

Shadow Dagger.
Melee Weapon Attack: +7 to hit, reach 5 ft., one target.
Hit: 8 (1d8 + 4) necrotic damage.

Shadow Claws.
Melee Weapon Attack: +7 to hit, reach 5 ft., one target.
Hit: 7 (1d6 + 4) slashing damage plus 4 (1d8) necrotic damage.

Eldritch Blast.
Ranged Spell Attack: +6 to hit, range 120 ft., one target.
Hit: 10 (2d6 + 3) force damage.

Shadow Step (Recharge 5–6). As a bonus action, Zathar can teleport up to 30 feet to an unoccupied space that he can see within dim light or darkness. Upon teleporting, Zathar can immediately take the Hide action.

Shadow Doppelgänger (1/Day). Zathar can create a shadowy duplicate of himself as an action. The duplicate is intangible and cannot deal damage, but it moves and mimics Zathar's movements. It has half of Zathar’s current hit points (rounded down). Any damage the duplicate takes is transferred to Zathar. The duplicate lasts for 1 hour or until it is destroyed.

Godly Resistance. If Zathar fails a saving throw, he can choose to succeed instead.

Shadow Dash. Zathar moves up to his speed without provoking opportunity attacks.

Doppelgänger Strike. Zathar’s shadow duplicate makes a melee attack. 

Shadow Veil. Zathar becomes invisible until the start of his next turn or until he attacks.

Dimensional Rift (Recharge 6). Zathar can tear open a rift to his home dimension. All creatures within a 20-foot radius of Zathar must make a DC 15 Intelligence saving throw or be pulled into the rift, taking 14 (4d6) psychic damage and being incapacitated for 1 turn. Creatures that succeed on the save take half damage and are not incapacitated. The rift closes after 1 turn.