I’ve always liked the Relictors (Chaos?) Space Marines Chapter, and the fluff I’ve found them in. I’ve read Atlas Infernal thrice blessed, and a Relictors Tech Marine duels a Khorne Berzerker … badass (no spoiler just blood)!
Here are some Relictors working for Inquisitor Zohar’el and double dealing with Chaos Lord Zoharion lolz. As the Blackstone Fortress in my Pacificus Sector is being explored, I’m sure they’re very interested.
I especially like painting my Space Marine minis as Relictors (even desecrating Ultramarine insignia to do so) and running regular Space Marines as Chaos, or vice versa … HERESY:)