The Sword of Hephaestus was a weapon that was forged by Hephaestus. It was one of few things that can pierce the Metal of Hephaestus. Whomever uses the Sword to strike an object forged by Hephaestus will perish after the blow.
Xena claimed the sword when Prometheus was captured by Hera and chained with the Chains of Hephaestus. She and Herakles fought over who should strike the blow to break the chains, but eventually the sword would fall on to the chains and break them itself, meaning neither of them had to lose their life.