Didn't the marauder horsemen kit go direct only recently?
Slaves to Darkness has some really dated models. The Chaos warriors need a re-sculpt. They are so iconic, but those models go back, I think, to the early 90's.
So I hope you are right. I also hope Slaves to Darkness gets a proper release.
Darkoath could easily be part of the Slaves of Darkness army. Could be battalion, allegiant, etc.