Caladbolg ("hard belly", or possibly "hard lightning"), sometimes written Caladcholg ("hard blade"), is the sword of Fergus mac Róich from the Ulster Cycle of Irish mythology. Spelled Caladcholg, it is also associated with the more obscure Ulster hero Fergus mac Leda, suggesting a conflation of two legends. It was said to be a two-handed sword that made a circle like an arc of rainbow when swung, and have the power to slice the tops off hills and take out several men.
During the Táin Bó Cuailnge, Ailill mac Máta takes Caladbolg away from Fergus mac Róich when he discovers Fergus' affair with his wife Medb. He gives it back when the Ulstermen rally against his armies. Fergus wreaks havoc against Ulster's forces with his blade, but Conall Cernach convinces him not to kill Conchobar mac Nessa. He redirects his blow above Conchobar's head, chopping off the tops of three hills in the process.
Caladbolg may be related linguistically to Cúchulainn's spear, the Gáe Bulg, and is thought to be a source or analogue of King Arthur's sword Excalibur, which in early Welsh is called Caledfwlch.
Care of Wikipedia. Excuse me if this weapon has already been submitted.