From the Super Mario Wiki, the Mario encyclopedia
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The origin of the Excalibur is unknown, though Mervin the magician seems to have some knowledge of it. The Excalibur is located in a lake in the Forest of Perpetual Night and Terrible Dangers. In "King Mario of Cramalot", the Excalibur is summoned when a bolt of lightning struck the lake it is hidden in; this lightning bolt raises a hand out of the lake, which proceeds to toss the Excalibur to Mario.
Later, Mario uses the Excalibur against King Koopa and his Golden Plunger; in the end, the Excalibur proves to be more powerful than the Golden Plunger and King Koopa is defeated. What becomes of the Excalibur afterwards is unknown.
The Excalibur seems to possess a wide variety of abilities, by simply touching it, Mario is transformed into Super Mario. The Excalibur can also switch between being very flexible to being stiff and sword-like. The Excalibur can also extend and be used like a grappling line.