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Cast from Hit Points if it always hurts the user
Living Battery if it's specifically a weapon powered by Life-Force or similar
Self-Damaging Attack Backfire if it hurts the user if it fails
Death or Glory Attack if it's powerful but severely harms or cripples the user, often even if it succeeds.
If you've got someone who specializes in weapons or attacks that harm themselves in addition to the target, you're probably looking at a Combat Sadomasochist, possibly also Improbable Weapon User and/or Impossibly Cool Weapon as well.
A weapon have chance to hurt it users