Bio: The Blenhims are a bit hard to categorize. They are not specifically a species, like the Daleks, but really more of a political movement, like the Nazis. They conquered several planets around the time of the Earth's 18th century. (Some of the inhabitants of these conquered worlds fled to Earth, and were physiologically close enough to interbreed with humans, making some residents of 20th century Earth part alien.)
The basic goal of the Blenhim Empire seemed to be racial purity and dictatorial rule. Leadership of the Empire could be challenged by any member of Blenhim society who dared challenge the current leader to physical combat.
The Second Doctor made a successful challenge of this nature, and then tricked the majority of the Blenhim military to fly into a star.