Kraarn

Kraarn of the Kraagaaron was a villain who once was defeated by an unknown incarnation of the Doctor. He later joined forces with other enemies of the Doctor to try and kill the renegade Time Lord.

According to Zagrath, Kraarn was the most evil being in the galaxy, 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. (DWM: Death to the Doctor!)