Excalibur is a mythology gag character created by Smash YouTuber Alpharad, in reality a simple recolor of Zetterburn. He is stated to be Alpharad's original character, allegedly the most powerful demon from the seventh circle of Hell who is far stronger than Zetterburn, likely as a parody of many similar OCs in other fandoms. He was featured in various pre-release videos, referred to in numerous inside jokes, and eventually released as a limited Premium Skin.
His Premium taunt involves displaying a sword. However, being a skin and not truly a separate character, his having one does not affect his moveset in any way.