The Forces of Destruction are united to "tear reality asunder". The conditions of the alliance, or whether they even know they are fighting as a united faction is currently unknown. None of the races of Dark Millennium have been fighting together as one unit.
Chaos forces are on the planet Sanctum. A number of cultists and World Eater Space Marines the world. The Cultists do not seem to be dedicated to a particular Chaos God but the "Enemies of Man" have been seen supporting a Great Unclean One, a greater daemon of Nurgle.
We is gonna stomp da universe flat and kill anyfink that fights back. We're da Orks, and was made ta fight and win.
(On a serious note: we have no idea.)