Thesoden was King of Rohan and was turned really old by Grima Wormtounge. The only reason Theoden did not die was beacause under Saruman's orders, Wormtounge placed a horcrux and Theoden. This would make the King of Rohan invincible and live for ever. He was turned back to normal by Erik Lehnsherr. Afterwards's he allied with Aragorn, son of Arathorn. He was crushed by his horse at the Battle of the Pelannor Fields. He survived due to his horcrux and in turn for saving his life, Theoden joined Saruman's Army. He secretely joined Voldemort as well and worked as a double-agent, secretly plotting to kill both Gandalf and Voldemort.