So Bill Cypher decides he needs a new body in the mortal realm for his plans. However having learned his lesson from Dipper, he decides he needs a tougher body. He settles on manipulating the nightmare of the CMC to try to crush their resilience and allow him to take over their body. Princess Luna sees this and isn't going to put up with this kind of sh** on her citizens. So the fight breaks out in earnest with the souls of three fillies on the line.

Bill Cypher vs the Princess of the Night Luna in the Dream-Realm.