This hand looks mummified and smells slightly of rot. 

In order to attune the hand, you must chop off your left hand. When pressed to the stump, the Talon of Degradation will painfully graft itself to the arm. Blood will flow into the hand, and after 10 minutes, the hand will look more healthy. The hand has sharp black claw-like nails and white pasty skin.

Once attached, the owner has full normal use of the hand. Melee attacks or melee weapon attacks made with the left hand are considered magical and inflict an extra 1d6 necrotic damage. 

The touch of the hand corrodes non-magical ferrous metals. If an item is not being worn or carried, up to 1 cubic foot of material is destroyed. If an attack is made with the hand against non-magical metal armor, it takes a permanent and cumulative −1 penalty to the AC it offers. Armor reduced to an AC of 10 or a shield that drops to a +0 bonus is destroyed. 

Notes: cutting off left hand.