Bonethief

Unusual Nature. The bonethief doesn’t require sleep.

Ambusher. In the first round of a combat, the bonethief has advantage on attack rolls against any creature it surprised.

Surprise Attack. If the bonethief surprises a creature and hits it with an attack during the first round of combat, the target takes an extra 10 (3d6) damage from the attack.

Possessed. The bonethief can burrow into an intact corpse it can see within 5 feet of it. The corpse must be Large or smaller and be that of a beast or a humanoid. The bonethief is now effectively the possessed creature. Its type becomes undead, though it now looks alive, and it gains a number of temporary hit points equal to the corpse's hit point maximum in life. It reverts to its true form if it dies.  In a new form, the bonethief retains its game statistics and gains the ability to speak, but its AC, movement modes, Strength, Dexterity, and other actions are replaced by those of the new form, and it gains any statistics and capabilities (except class features, legendary actions, and lair actions) that the new form has but that it lacks.

Multiattack. The Bonethief can make two melee attacks.

Arm Blade. Melee Weapon Attack: +3 to hit, reach 5 ft., 1 target. Hit: 7 (1d6 + 3) [slashing] damage. 

Lunge. Melee Weapon Attack: +3 to hit, reach 15 ft., 1 target. Hit: 16 (3d6 + 5) [slashing] damage. A creature is also grappled by it (escape DC 16). Until this grapple ends, the target is restrained.

Gouge Out. If a creature has been grappled by the bonethief, than it can attempt to burrow inside the creature. The bonethief makes a Melee Weapon Attack: +5 to hit at advantage. Hit: 31 (5d10 + 3) [piercing] damage. If the creature drops to 0 hit points, it is instantly killed and the bonethief obtains the possessed property.