Complex sentence: T'yog created a scroll that was supposed to protect him from the petrifying effect of gazing upon Ghatanothoa, but was defeated after Ghatanothoa's priests stole the scroll and replaced it with a fake one.
Split into simpler sentences: T'yog created a scroll that was supposed to protect him from the petrifying effect of gazing upon Ghatanothoa. But T'yog was defeated after Ghatanothoa's priests stole the scroll and replaced it with a fake one.