When Forrest Kills an enemy, he can use his reaction to make a Sleight of Hand check (DC = 8 + CR of Creature). On a success, he steals their soul as it departs from the creatures body. When this happens the Thief of Nine Lives takes on an appearance that superficially resembles the felled enemy. Until the end of the next round, Forrest can use an action or ability belonging to the creature (DM's Decides). If Forrest were to gain an ability that requires a saving throw, the DC = 8 + Proficiency Bonus + Wisdom Modifier. 

Alternatively, Forrest can consume this soul to bolster his vitality. Upon consumption, Forrest gains 1d10 temporary Hit Points and has advantage on the next saving throw of an ability that creature was proficient in. (For example, if Forrest killed Bag of Nails he'd have advantage on his next Dexterity saving throw.)

Forrest can use this feature a number of times equal to his proficiency bonus.

The hand also possesses other lingering properties, namely, the ability to imitate the powers of others. These martial-arts take on the form of those they are imitating in the style of fists.

Fist of Loam and Root. Forrest's paw becomes loamy, with bits of foliage poking out here and there. His claws, sharp as thorns. While using this form, you can cast Greater Restoration 1 time, requiring no material components. 

Fist of Caustic Divergence. Your paw becomes coated in bright green, goopy liquid. While channeling this fist, your attacks deal and additional 1d4 acid damage. As a bonus action, you can can spend 1 ki point to make a ranged attack and spew poisonous darts at an opponent within 30 feet of you. On a hit, these darts deal 4d4 acid damage. Any target hit this way becomes coated in corrosive poison, dealing 1d4 acid damage at the beginning of their turns. To end this effect, a creature must spend it's action to wipe off the poison. At the end of each of your turns, you take 1d4 acid damage, as the corrosive substance begins eating away at your yet living flesh.

Fist of Brilliant Tyranny. Your fist becomes clad in brilliant golden plumage and black, hooked talons. While using this form, your unarmed strikes deal an additional 1d4 radiant damage. As a bonus action, you can cast Misty Step at will, disappearing and reappearing in a flash of brilliant light Any creature who is within 5 feet of Forrest when he reappears must make a constitution saving throw. On a failure, they are blinded until the end of their next turn. Once they save against this ability, they are immune to it's effects for 24 hours. Afterwards, you take 1 level of exhaustion.

Notes: Tabaxi daring (dumb) enough to steal treasure that's obviously booby-trapped.