Complex sentence: The mages betrayed Nimeth when they came across the Scroll of Nogran, which promises wealth and power for committing the described rite and opening a gate to forbidden dimensions.
Split into simpler sentences: The mages betrayed Nimeth when they came across the Scroll of Nogran. This scroll promised wealth and power to the person who commits the rite described in, while in fact the rite opens a gate to forbidden dimensions.