So while scouting for the Mandalorians, the group met their enemies. The 501st were ready to patrol an area and the players met them. Surprised slightly that the troopers had blue bladed light sabers and were immediately on them as their shuttle landed, demanding answers. Of course the group tried to lie, which only lasted a while. They began hearing portions of history. "When Vader left the empire..." "Wait... what?!"
Slowly they found out that the Sith Prophet took over behind the scenes and made Palpatine his puppet. Yoda then came out of hiding to strike back, turning Vader back to the light side and with him came the 501st.
Starkiller survives and helps the Empire while the three Skywalkers, Kenobi, Yoda and a few others have taken residence in the Executor, creating a Jedi academy in space. Now the 501st are a group of new recruits, former 501st and many Jedi in training.
The Mandalorians have been expanding with the chaos caused by such powerful forces clashing. Taking over in the wake left by the battles. Their hit on the Executor was to be staged while the Rebels were destroying the Death Star, there by taking out a major piece of both the Empire and the Rebels.
The Rebels have a fairly large army with the atonement of Vader. The young Skywalker children have been training and are fairly competent Jedi, thanks to Kenobi and Yoda. This lets them operate in the open more than they would have been able to before. Thanks to this they will be able to make a hard hit on the Empire.
The Empire has a more overtly violent bent with Starkiller killing every Jedi he can find, which is easy thanks to the farseeing powers of the Prophet.