The Flawless Host
"We are the purest of all. We can do no wrong."
- – The catechism of the Flawless Host, the humblest known phrase ever uttered by a Slaaneshi warband
The truly hilarious Chaos Space Marine warband known as The Flawless Host is an example of what happens when you select Slaanesh, God of Hubris and Desire, to be your patron deity. Believing themselves to be the embodiment of justice and purity, it seems they are so drugged up they failed to notice all the screaming victims they leave in their wake and the mutations that spring from their bodies. After all, if it happens then it is merely another example of their godly-approved purity/justice/virtue etc etc etc.
Once they were a loyalist Space Marines known as the Shining Blades, whose gene-seed was pure and free of taint, they had a rigid mental training regime and they kept winning so many victories they were developed an unshakeable faith in their abilities and were convinced they could do no wrong. Hmmm, that sounds vaguely familiar....
Anyway, to the complete surprise of no-one, they developed such an overwhelming pride and self-delusion they attacked anyone that even raised the slightest objection to their actions. They are basically that drunk girl at the club that can't take the slightest criticism without breaking down and calling you a bitch, or your average Tumblr user. (The club? Women? You think too highly of us, Brother.)
After deciding that the Imperium were ungrateful slobs for not recognising that the Shining Blades were the best thing ever, they renamed themselves the Flawless Host, dedicated themselves to Slaanesh for all those screaming daemonette fan-girls and went on a tour of the Imperium scarring it,and having their own pride parade, waving banners and screeching how perfect they are. In case this slipped you by, other Chaos Marines consider them preening self-absorbed dicks and only tolerate them as Slaanesh would throw a hissy fit.
Once they invaded an Imperium mining planet for the sin of not being beautiful enough. Then the Legion of the Damned torched their daemonette groupies, causing them to run away because they were worried they'd break a nail. They invaded the Imperial world Sendennis for similar reasons (paragons of Justice they are, cleansing the galaxy of ugliness) but the Carcharodons showed up to stop their righteous crusade.
Notable for having one of the coolest names of any warband, contrasted with one of the worst colour schemes. Purple and pink. Not the best. The irony is possibly intentional (by Games Workshop).
Infamous for one of their attacks which killed Billions on a Hive World (Must be competing with the Purge and Skullsworn on who gets the most kills in short amount of time)
Basically canonical Chaos Pretty Marines.