Sword of Sig
The Sword of Sig is an incarnation of the sword Excalibur, given to the LNH's incarnation of King Arthur, Sig.Lad. Like all things Arthurian, it is a legend that has taken different forms in different stories:
In Electrocutioner's Song, Merlin Ambrose gave the Sword of Sig, also known as the Sword of Change, to Sig.Lad, stabilizing his powers when they threatened to go out of control and dissolve him. Sig.Lad wielded the Sword of Sig in battle against Acton Lord several times. When Acton Lord took command of the Multiversal Office, Sig.Lad thrust the Sword into its terminal, locking down its higher functions. Sacrificing the Sword rendered him vulnerable to his powers being corrupted again, with tragic consequences (appropriately, for an incarnation of Arthur).
After Sig.Lad's death, Acton Lord took on a new life as Kid Pocky. He summoned the Sword of Sig to finally defeat Darkheart and Steelwind, proving his worthiness as a hero. sig.ma appeared to him and offered the choice of whether or not to keep it, telling him he could restrain the Office's power without it, but Kid Pocky decided the sword didn't belong to him.
sig.ma evidently returned the Sword to the Office, as when Library Lad modified Looniverse's Universal Office, he declared that the one who was worthy to draw the sword from the stone could be the Administrator, and that he was playing the role of the Lady of the Lake...
A family of related swords exists in this world as well, including Nerf Girl's Sword in the Stone (in the form of a Nerf sword) and the Sword of Damocles, the sword against kings, Excalibur's equal and opposite.
It's sometimes spelled as the Sword of .Sig or .sig; see sig.file.