Emperor's Children
They worship Slaanesh, wear pink and black, and were part of the original first founding legions. They are the weird fat girl who has rape fantasies that no one will ever talk to because she's far too weird. There's always the temptation to be nice to that fat girl because you know she's desperate enough for attention to do some pretty freaky stuff if you asked that normal girls would dump you for.
NOT REALLY CHILDREN OF THE EMPEROR. They were unswervingly loyal way back when, which is how they got their name, and now they just use the name as a form of mockery. Their Primarch, Fulgrim, was best buds with Horus, so when Horus did his heresy thing, Fulgrim went straight to Horus and said "Dude; what the fuck? For reals." Horus knew that Fulgrim was a perfectionist with mild OCD and, straight out of an 80's afterschool special, Horus convinced Fulgrim that he could get that perfection if he tried some of these diet pills... and meth... and speed... and crystal... and by the time Fulgrim was on cocaine he did a 180 turn and devoted himself and his legion to Slaanesh.
During the siege on Terra, the Emperor's Children got bored, broke off and went pillaging the civilians. Most of the innocents they found were ground up and distilled into drugs, anyone left over was raped to death. After the Siege, they dragged a shittonne of slaves with them back into the Eye of Terror. After snorting/smoking/raping/defenestrating/dogwielding them all up, they started stealing slaves from the other Legions, which started all the in-fighting. They did manage to steal Horus's corpse, and made at least one evil clone which they raped to death later.
Marines of the Emperor's Children legions are always looking for the next high, and eschew tranqs and depressants (like booze) for hallucinogens and stimulants (like cocaine). It started out as a quest for perfection with performance-enhancing drugs, but now that they're full-blown corrupted by Slaanesh it's about peak experiences. Also have an obsession with noise, furry porn, tentacles, and gettin' high. Hey, who's the bastard who forgot about their chronic masturbation problem?
The Pre-Heresy paint scheme for their minis are purple and gold, while the Post-Heresy paint scheme is pink and black. (Or pink and any clashing pastel colours.)
Among the most infamous members of their Legion are Lucius the Eternal, Fulgrim and Trap Marine.
It is speculated that the Pretty Marines' unknown Primarch was actually an Emperor's Children Captain that remained loyal to the Emperor during the Horus Heresy and convinced the High Lords of Terra to allow him to re-create the example of human flawlessness using his own gene-seed. Hence it would greatly ac....*Angry Marine shoves the writer aside and takes over the keyboard* GODSDAMNED FUCKTABULAR ACTIONS THAT GETS ON MY NERVES. REALLY THESE 2 PRISSY FAGGOTS GET ON MY FUCKING NERVES, EVEN MORE THAN THOSE MORALFAGGOT ULTRAMARINES.