Question: why did emrakul trap herself?
Answer: Phyrexia would be a hell of a set. This theory kinda fringes on Emrakul being "brought" to Innistrad, or even lured by the corrupted leylines triggering a false sign that the plane was dying. ... TLDR: Emrakul put itself in the moon in order to be transported by Karn to fight Phyrexia.