Complex sentence: He briefly tells her that she is the Slayer and she kills the demons after slaying the demons attacking her wedding, Harly walks out on her and she knows her life won't ever be the same.
Split into simpler sentences: He briefly tells her that she is the Slayer and she kills the demons. But after Mollie slays the demons attacking her wedding, Harly walks out on her and she knows her life won't ever be the same.