Complex sentence: He assisted Sauron in the War of the Ring, biding time to secure the One Ring for himself, in order to supplant Sauron, but his plans and forces were thwarted by the power of the Ents.
Split into simpler sentences: He assisted Sauron in the War of the Ring, biding time to secure the One Ring for himself, in order to supplant Sauron. But his plans and forces were thwarted by the power of the Ents.