It should be obvious that this is what happened.
Sylvanas was a simple banshee when she died. She was not a special creature. However, she was easily manipulated by the Jailer, who played into her terror of the Maw and desire to create a better afterlife for herself and everyone else.
Of course, the Jailer doesn't care about mortals controlling their own fates, he just wants to consume everything. That's a convenient lie he used to lure Sylvanas into his side. The pieces are easy to put together.