What about the Soul Cube from Doom 3? I know 3 was pretty bad as far as the Doom legacy's concerned, but it could shape up to be an interesting unique weapon, and could even be the reward for a unique level.
It could be a usable inventory like the Hell Staff, and function similiarly: when you use it, it causes damage to a single enemy, the same amount of which is given to you as health, although not over their (or even your, as in 100%) maximum. If it had a damage rating of, say, 5d5, it could potentially kill or at least injure most enemies up to the late-game, and potentially heal you a full half of your maximum health, making it good for sketchy situations. And like the Hell Staff, it makes you Tired on use, to prevent abuse.