Complex sentence: Gaheris Rhade had betrayed his oath and killed his best friend in an attempt to protect the Nietzschean people from the terrifying race of the Magog, but instead he had left the galaxies defenseless against the Spirit of the Abyss.
Split into simpler sentences: Gaheris Rhade had betrayed his oath and killed his best friend in an attempt to protect the Nietzschean people from the terrifying race of the Magog. Instead, he had left the galaxies defenseless against the Spirit of the Abyss.