[Heroka Vendile]

Dark Mechanicus -
Pretty much the same as the AdMech, but are baically behind the creation and invention of all daemon engines and most renegade vehicles, ie things like the Defiler, the Plaguereaper and Abaddons infamous flagship the Planet Killer.
They maintain the chaos titans, vehicles, technology, ships, etc, etc.
They are also just as political and in-fighting as the AdMech, if not more so.
They are also one of the possible sources of where the Obliterator Virus came from.
They don't use AI, as even tho they suffer none of the imperial restrictions, they can instead possess their creations with human souls or daemons.
They take advantage of the choas powers as much as worship them.
Some become more like sorceerers than mechanics.

But yeah, there isn't much official about the Dark Adeptus out there really. Is would imagine Seige Of Vrakks part 1 & 2 will probably have a small amount at least.