Theoden son of Thengel was the King of the Mark (Rohan) at the end of the Third Age. He was by nature a just and strong-willed ruler, both stern and wise. His chief mistake was in letting Grima Wormtongue become his most trusted advisor and, gradually, replace the King in his capacity. Grima’s secret goal was, of course, to weaken Rohan and to render its army as useless as possible so that Saruman would have an easier victory over it.
While Grima’s lies lingered in Theoden’s mind, the King grew old and frail. He was awakened and healed by Gandalf the White, who uncovered Wormtongue’s treachery. Later Theoden defended the stronghold at Helm’s Deep and rode to the defense of a besieged Minas-Tirith at the head of the army of the Mark. There he was slain when a Nazgul beast gripped Snowmane, Theoden’s horse, in its claws and then hurled him on the ground. The King was avenged by Eowyn, his niece. The Ride of the Rohirrim and the King’s valiant death have kept the bards and minstrels of Middle-earth busy for many subsequent years. Among the songs and epitaphies composed on Theoden’s death was this one:

Out of doubt, out of dark, to the day’s rising
he rode singing in the sun, sword unsheathing.
Hope he rekindled, and in hope ended;
over death, over dread, over doom lifted
out of loss, out of life, unto long glory.