ooooh do go on!
I know it's cliche now, but the Fallen Sororitas has always been a 'thing' for me, though I could rarely find an instance, personally, when I'd actually go, "Well yah, I can see her falling." And by Falling I mean willingly falling, not this mind-control type stuff that's been going around in the novels recently. But then I heard about "The Khorn Knights". For those unfamiliar, in the 40k universe there are 2 factions in the Imperium of Man who are seen as beyond reproach when it comes to falling to corruption. One is the Battle Sisters, the Adeptus Sororitas. Women baptized in the ultimate faith in the God Emperor of Mankind who can perform legit-miracles and the chapter of the Adeptus Astartes called The Grey Knights, who are the anti-chaos marines. By their very nature they're immune to chaos, completely. In lore and in the game mechanics Grey Knights can not, ever, be touched by chaos corruption let alone succumb to it. However in one book written by the dreaded Matt Ward, he had a group of Grey Knights come across a source of corruption on a planet, and fearing they would be corrupted by it (for some reason), they went to the local Sororitas chapter, slaughtered them, desecrated their bodies and then adorned their power armor in the blood of the slaughtered sisters to make especially powerful wards before venturing after the source of the corruption. And that's it. They were the main characters of the story, the story progressed and in the end the Imperium was OK with what happened. No repercussions.
My whole thought was, "What if one sister survived who saw it all?" and in my mind that would be more than sufficient for a sister to fall, willingly, to chaos after seeing the Emperor's Angels defile her sisters. So I wrote the little story background and decided in the end the surviving sister would end up picking up a straggler grey knight and disabling his armor's generator, rendering him immobile and torture him, trying to get a confession from him that he was chaos etc. He of course refused for days of intense torture, interrogation taken to excess, but she never stopped. She began to sing and after having watched from the great unknown the Ruinous Powers reached out and filled her lips with a song to chaos, and as she continued her administrations to the Astartes the old covenant was swallowed into the warp by an unknown force. The sister gifted the still-living grey knight to Slaanesh, and in return he made her more powerful than she'd ever been. Being a fallen Sororitas made her a metaphorical rock-star in the Vortex, and she built a warband and an armory based on her celebrity status. She developed a fondness for sonic weaponry after relating her song of chaos that summoned the warp to take her away to a cleansing wave of sound that she wanted to share with all of her sisters, to show them the truth of what happened that day, having kept the recording her armor made of that day.
Originally made for a space-faring game, since she had the most social and command skills, she was elected OOCly to be the owner and captain of a modest ship on which the rest of the PCs would be. We would travel around the vortex and the verse and collect weapons, artifacts, attack outposts, gather followers and unique individuals etc etc. During the course of play it was discovered (aka the GM and I just decided on it after a few exchanges) that my character and the ship's AI were in a relationship of sorts. Physical mostly, but they liked each other... so basically my character was fucking her own space ship. She also had a 'thing' for different random sexual relationships that would come up randomly. Like, a random demon would show up because someone summoned it, and the GM would have the daemon remember my character and we'd just run with it and spontaneously create some crazy story about how they met once and did something Slaaneshi-crazy.
She's had a few versions played in different games. One game I gave her "Cursed Heirloom", a talent in the game that gives you a piece of expensive gear, but it's cursed and you can't get rid of it no matter what you do, and of course there's a downside. Like, once a session you're forced to reroll a skill check pertaining to the item etc. Well I decided, sneakily so, that she had a cursed motorcycle. It could blow up or be left on a planet and suddenly it's there in the cargo hold the next day. I rationalized it that she was once sexually assaulted by the daemon prince Doomrider, taking only a few seconds to happen but those few moments she remembered so fondly and with such high drug and adrenaline fuel that she became convinced that they were now dating... no matter what the daemon prince would say in protest. The bike the 'assault' took place on was cursed and bound to her. It's chaos, try not to think too hard on it. Lol
But yes, she uses heavy sonic weaponry, noise cannons and blasters as well as her old Sororitas gear when necessary. She loves revealing the truth of the Imperium and the Astartes to fellow sisters, and her goal is to convert as many sisters as she can to chaos. While she'd love to break the Imperium as a whole, she's more goal-orientated and not really a 'big picture' kind of woman. She wears her old Sororitas power armor, but it's been embraced by Slaanesh, so it looks like the stereotypical mid-riff exposing battle armor but still protects as well. She's probably one of my strangest characters to date in any game system, but probably one of my favorites. :)