Kaladbolg ("hard belly" or "hard lightning") is the sword of Fergus mac Róich from Irish mythology. It was said to be a two-handed sword that made a circle like an arc of rainbow when swung, and to have the power to slice the tops off hills and take out an entire host.
During the Táin Bó Cuailnge, Ailill mac Máta takes Kaladbolg away from Fergus mac Róich when he discovers Fergus' affair with his wife. He gives it back eventually, and Conall Cernach convinces Fergus not to kill Conchobar mac Nessa. Fergus strikes the Three Great Strokes on three small hills instead, blasting off their tops.