Yarsith, the Mage of Eyes

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

Spellcasting. Yarsith is a 6-level spellcaster. Its spellcasting ability is Charisma (spell save DC 16, +7 to hit with spell attacks). Yarsith has following spells prepared:
Cantrips (at will): Chill Touch, Eldritch Blast, Fire Bolt, Poison Spray, Thunderclap, Vicious Mockery 
3rd level (2 slots): False Life, Mage Armor, Misty Step, Burning Hands, Command, Counter Spell, Darkness, Fireball, Hellish Rebuke, Misty Step

Dagger. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d4 + 4) piercing damage. Range if thrown 20(60).

Spellcasting. [at will] Chill Touch, Eldritch Blast, Fire Bolt, Poison Spray, Thunderclap, Vicious Mockery 

Can cast Misty Step at level 2. It will use a spell slot.

Opportunity Attack

Spells. Hellish Rebuke (1st), Counterspell (3rd)

Call to the Dark One (2). Yarsith calls upon the god Yakarn for aide. He summons 1d4-2 (minimum one) Mane.

Dark Descent (2). Yarsith magically descends to the underworld and then immediately reappears 40 feet from where he discended.

Essence of the Dark One (3).  Yarsith prays to Yakarn. The dark god grants his servant a new open spell slot. This Legendary Action cannot be done again until two other Legendary Actions have been done.