The Sword of Doom is an ancient magical artifact.
It was made by Masamune's evil-minded apprentice Muramasa for a Ronin. When the Ronin drew the blade from it's scabbard, the evil spirit of the apprentice that had passed into the blade, possessed the Ronin, becoming the Black Samurai. Fortunately, Masamune created the Sword of Fate, and gave it to a Green Dragon. The two swords fought an epic clash, until the Green Dragon defeated the Black Samurai, imprisoning him within the Sword of Doom. It was the key to bringing the Black Samurai back to life, so it was hidden by the Green Dragon.
Scooby-Doo! and the Samurai Sword
The Gang was tricked into finding it and retrieving it by the cunning Miss Mirimoto. She succeeded in reviving the Black Samurai, but refused to serve her, and plans to rule the world forever. At that moment however, Shaggy and Scooby arrived on the scene with the Sword of Fate. The duo dueled the Black Samurai taking turns using the Sword of Fate. In the end, Scooby destroyed the Sword of Doom, by slicing it in half. Doing so, purged the evil within the broken, freeing the possessed Ronin, who died in peace.