Fallen Knights

Army concept: evil Grey Knights. Sort of the chaos version of them. Dark and splendorous.

Use GK list with new GK models but painted black, gold and red. Maybe carve or scratch out their lettering and give them demonic back banners instead of Pauldrons.

Or maybe just make (finally) an all Dark Paladin army using chaos terminators.

Subsidiary models, like the Weaponsmith would be dark versions, mutants and hideous aliens. Or cowled figures, machinists.

Oooh, or pre-heresy thousand sons variant integrating soon-to be released tomb kings pieces. Warsphix as Dreadknight. Tall and stately guardians of ivory pyramids as Terminators.

Maybe this bad girl as a storm raven.