Courix, ghostly champion of darkness

Shadow Sight. Courix can see 100 feet into the shadowfell when he is on the Material Plane, and vice versa.

Incorporeal Movement. Courix can move through other creatures and objects. He takes 5 (1d10) force damage if he ends its turn inside an object unless he can possess it.

Multiattack. Courix makes two life draining sword attacks.

Life draining sword. Melee weapon, +9 to hit, reach 10 feet, 1 target. Hit: 18 (4d6+4) Slashing damage plus 7 (2d6) necrotic damage and the target makes a DC 15 CON save or have their HP maximum reduced by the amount of necrotic damage dealed.

Dark return. Courix enters the shadowfell from the Material Plane, or vice versa. 

Possession (Recharge 6). One humanoid that Courix can see within 5 feet of him must succeed on a DC 13 Charisma saving throw or be possessed by him; Courix then disappears, and the target is incapacitated and loses control of its body. Courix now controls the body but doesn't deprive the target of awareness. He can't be targeted by any attack, spell, or other effect, except ones that turn undead, and retains his alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. He otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.

The possession lasts until the body drops to 0 hit points, Courix ends it as a bonus action, or is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, Aurion reappears in an unoccupied space within 5 feet of the body. The target is immune to Courix's Possession for 24 hours after succeeding on the saving throw or after the possession ends. While possessing a creature, Courix retains his sunblade and chain mail, which appear on the target.