Incorporeal Movement. The spirit 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.

Soul Bond. The following numbers increase by 1 when your proficiency bonus increases by 1: the spirit’s skill and saving throw bonuses above, and the bonuses to hit and damage of its Sapping Grasp.

Sapping Grasp. Melee Spell Attack: +4 to hit, reach 5 ft., one target. Hit: 7 (2d4 + 2) necrotic damage. 

Otherworldly Possession (Recharges after a Short or Long Rest). One creature the spirit can see within 5 feet of it must succeed on a Wisdom saving throw against your spell save DC or become charmed by the spirit ; the spirit then disappears, and the target is incapacitated and loses control of its body. The target is now under your control, and you can use your bonus action while the target is possessed this way to command the target to take any action that you could command the spirit to do on its turn. While the spirit is possessing a target this way, it cannot be targeted by any attack, spell, or other effect, except ones that run undead.

The possession lasts until the possessed creature drops to 0 hit points, you end the possession as a bonus action, or the spirit is turned or forced out by an effect like Dispel Evil and Good spell, or until 1 hour has passed. The target may repeat the saving throw at the end of each of its turns, ending the effect on a success. Additionally, on a success, the target is immune to this feature from any spirit you summon for the next day.  

Absorb Harm. When a creature the spirit is possessing takes damage, the spirit may use its reaction to negate all the damage and take psychic damage equal to the amount of damage that was negated.