Slitheen is the surname of a family of corrupt, greedy criminals, from the planet Raxacoricofallapatorius.
They are descendants of the Slavereen, the earliest descendants of the Huspick Degenerate
All they care about is money, and how to get it, by any means necessary.
They are notorious for landing on a planet, killing overweight members of the dominant species and disguising themselves inside the 'skin-suits', using compression field technology collars around their neck
Originally, Raxacoricofallapatorians lived in peaceful democracy, until the Slitheen bribed their way into power. Their rule crashed the economy, then the rest of the families rose up against them and the Slitheen were exiled. Some 'faced justice', but some branches escaped and fled to the farthest corners of the galaxy.
The Slitheen first appeared in (Aliens Of London/World War Three), where the Pasameer Day branch, led by Jocrassa Fel Fotch Pasameer Day Slitheen, planned to infiltrate the country, disguising themselves as various humans holding leadership positions, attempting to start a nuclear war. They wanted to turn the Earth into a huge nuclear reactor pile and sell its remains off as starship fuel. This attempt was foiled by the 9th Doctor, when he ordered Mickey Smith to shoot down Downing Street with a missile, killing the Slitheen and saving the world
However, Blon Fel Fotch Pasameer Day Slitheen managed to teleport out in time. She arrived on the Isle of Dogs. Later the same year, Blon, still in her Margaret Blaine suit, managed to get herself elected Mayor of Cardiff, and tried to abuse her position in an attempt to escape being stranded on Earth by destroying a local nuclear reactor. The power from the Rift, situated in Cardiff, would amplify the explosion so it would not only destroy Earth but enable her to escape the planet safely on a tribophysical waveform macro-kinetic extrapolator. This effort was also foiled by the Doctor and his allies when the Doctor used his sonic screwdriver to repeatedly reverse Blon's teleport. When she tried to use the TARDIS as a secondary explosion, Blon was reverted into an egg, by the heart of the TARDIS, and returned to her home planet to be given a second chance at life