The Dark Claw is a weapon mentioned in the Doom Bible. The description says: "Demon weapon that casts a dark cloud of tortured essence. Silent but deadly. The Dark Claw feeds on human souls. Killing possessed humans or hellslaves allows the weapon to feed".
Like the Unmaker, this weapon never made it to the finished game; while the Unmaker made it to Doom 64, the Dark Claw did not.
An image of a severed, red hand can be found in the pre-release Alpha 0.4 and 0.5 WAD files. Whether this is the actual graphic file for the weapon remains to be proven, although there exists precious little evidence to support any other hypothesis.
A HUD overlay image from the first Alpha 0.2 version contains a mention of a "Captain's Hand". Game programmer John Romero has said the item--along "Heart of Lothar", the other item mentioned--was only meant to be a special key, reflecting the story outlined in the Doom Bible. In the following Alphas the overlay was gone. No further mentions of these items can be found.
Draining life energy from enemies was a design decision that was discarded in the later stages of game development. However, the concept was used in Doom 3, in the shape of the Soul Cube and in Resurrection of Evil in the shape of The Artifact. A "vampiric powerup" can also be found in the Alpha versions, up until the very last versions prior to release.