Yu☆Gi☆Oh! Duel Monsters

Synopsis

Mahad unlocks his full power using the Millennium Ring, and forces Bakura the Thief King to face him by trapping them both inside the academy where magicians used to be trained (inside of a cavern). Mahad draws upon his Millennium Ring to completely unearth his full powers, which he had sealed away within the academy for his own safety. Mahad's Illusion Magician faces off against Bakura's Diabound, which has grown in strength since his battle with the Pharaoh. However, Mahad's Illusion Magician has also grown in strength since then, but Bakura's Diabound is still the strongest monster on the field. Diabound gains the upper hand, and Thief King Bakura finishes the job by deliberately setting off the tomb's traps. To save himself, Mahad fuses his soul with his spirit beast, the Illusion Magician, transforming himself into the Dark Magician, which made him much more powerful than Bakura's Diabound, much to Bakura's disbelief. Bakura the Thief takes the Millennium Ring, however, Mahad as Dark Magician is able to destroy Bakura's Diabound, which causes the cavern to collapse. Thief King Bakura barely manages to escape, while Mahad's soul is sealed into the stone tablet meant for Diabound.

(Source: Wikipedia)