Shadow Melt. The Convert melts into a shadowy substance allowing it to move freely without provoking opportunity attacks.

Innate Spellcasting. The Convert’s innate spellcasting ability is Intelligence (spell save DC 17, +7 to hit with spell attacks). The Convert can innately cast the following spells, requiring no material components:
At will: Toll The Dead, Word of Radiance
2/day each: Maddening Darkness, Cause Fear, Thunder Step

Multiattack. The Convert makes three attacks: one with its lunge and two with its claws.

Claws. Melee Attack: +9 to hit, reach 5 ft., 1 target. Hit: 17 (#1d12 + 5) piercing damage. 

Lunge. Melee Attack: +7 to hit, range 10 ft., 1 target. Hit: 16 (1d12 + 4) bludgeoning damage. The target must succeed a DC 12 Strength saving throw or be stunned until its next turn.

Mind Tremor. The Convert exudes an aura of darkness and screeches in an ear-piercing pitch. Any creature within 100 ft. of the Convert must succeed a DC 14 Wisdom saving throw or take 2d10 psychic damage. Also on a failure, the target is frightened until the start of its next turn. This ability recharges whenever the Convert goes into its Shadow form.

As a bonus action, the Convert can melt into its Shadow shape and move up to 10 ft. This ability recharges after the Convert deals at least 30 damage.

Side Step. When a creature within 5 ft. makes a melee attack, the Convert jumps sideways 5 ft. on a successful DC 13 Dexterity saving throw without provoking an opportunity attack. On a failure, the Convert takes an additional half of the damage dealt to it.