After Alexander the Great acquired the Millennium Ring, Shadi tested him by making him go through the Capsule Monsters trials. He failed, and the ring split his soul in half, with one his evil side, and one his good side. This evil side, Yami Alexander, was sealed in a pyramid, and found Dr. Alex Brisbane, the direct descendant of Alexander, as a host.
Deceiving Yugi Muto and his friends, Yami Alexander had them go through the Capsule Monsters trials to unlock the power he could not obtain thousands of years ago. When the trials were finished, Yami Alexander raced to the room containing the power. Although Yugi and his friends protested, Shadi appeared and revealed that Yami Alexander did have a rightful claim to the power, having completed the trials long ago. However, Shadi allowed Yugi to challenge Yami Alexander for the right to the power.
Yami Alexander then called forth the spirits of his lost vassals to do battle, though he simply sacrificed their monsters to power his "Seven-Armed Fiend" and "Reshef the Dark Being". He saw them as mere tools for his own gain. Eventually, Yugi defeated Yami Alexander using the Armor of Unity, and he was destroyed, allowing his good half to happily pass on.
Yami Alexander is also one of the few characters in the Yu-Gi-Oh! series shown to be able to merge with his monster, namely "Reshef the Dark Being". Other characters with this ability include Yami Marik, Paradox, Aporia, and Z-ARC.