The legendary sword of King Arthur. Agents Lattimer and Myka bagged it in Wales, almost at the cost of Myka's life. She was engaged in a sword fight and was nearly killed until Pete threw her a stone laced with the same metal as the blade. When the sword and stone come in contact, the blade goes out of phase for several seconds, allowing it to pass through ordinary matter, which is how King Arthur was able to pull the sword from the stone.
In the original Arthurian myths, Excalibur was acquired by Arthur from a lake after the Sword in the Stone broke. However, there are many times in fiction (this being one of them) that the two swords are conflated.