Dusk is pissed off because his number 1 assistant is killed by Queen Umbra. Queen Umbra uses his rage in order to make sure someone strong enough takes her place as ruler of Equestria. He becomes a tyrant just like her, but more powerful. He seizes control. A year later of fighting with Dusk's forces Solaris has had enough. So as the saying goes, in order to beat a monster you have to make a monster. Umbra had already made a monster for them. After this then what happens next.