Excalibur is the legendary sword weilded by King Arthur according to Arthurian legend. The sword has had several names over time, and is thought to be related to Caladbolg, the powerful sword originally weilded by Fergus mac Roich. Not only does the blade have powerful abilities, such as blinding ones' enemies, but the scabbard itself is indued with powers.
This powerful sword is currently in the possession of rei.bot.
More information about Excalibur is available here.