Complex sentence: The Masters have found their most powerful magics to reside in the deaths of the Wyverns, which lead the Wyverns to ally with the men of the Argonath to fight against the Masters.
Split into simpler sentences: The Masters have found their most powerful magics to reside in the deaths of the Wyverns. This is the reason for the allinance between the Wyverns and the men of the Argonath to fight against the Masters.