Katsu

Katsu was a Sontaran who had a reputation for being the fiercest of all.

History
Katsu was a fierce warrior who was victorious in a thousand encounters and like all Sontarans sought battle. However, it was on the planet Keth that Katsu lost a battle against a Levanaxus in which he had to escape in his ship and it haunted him every day since then.

To make sure he'd never lose again, Katsu sought the ultimate weapon to make him invincible. His quest brought him to Aleph-777, a planet known for its tranquility, where the inhabitants relied on the ultimate armament. Katsu started killing the people, saying that he would not stop until they gave him the weapon. Eventually, one man escaped off-planet with the weapon, but Katsu pursued him.

Katsu tracked him to the planet Omega-666, where the man was apparently trying to hide the weapon. Katsu pushed the man aside and dug up the place, but instead pulled out a metal flask. The man used reverse psychology, telling Katsu not to open the flask, and when he did, Katsu was covered in the Omegan Green Plague which the flask contained, and slowly dissolved away. (DWM: The Final Quest)