Baalor's Claw

Baalor's Claw can be used by Cespenar in Baldur's Gate II: Throne of Bhaal. It can be used to upgrade weapons which are enchanted by souls of evil creatures or items which can be wielded only by evil-aligned characters. It's found in the Marching Mountains temple, it's dropped by Fire Elemental Prince.

In-game description
"This demon's hand pulses with evil power. Though harmless enough in its present state, there are rumors that the magic of a demon's claw can be unleashed by combining the appendage with another powerful item of evil."