Kraarn

Having once been defeated by him, Kraarn of the Kraagaaron joined forces with other enemies of the Doctor to try and kill the renegade Time Lord.

According to Zargath, Kraarn was the most evil being in Mutter's Spiral and had laid waste to whole solar systems and enslaved entire species.

Just before launching an invasion on Mercury, he involuntarily caused an electrical surge, which killed him on the spot. (COMIC: Death to the Doctor!)