Théoden was the son of King Thengel and Morwen of Lossarnach. He became the 17th King of Rohan. Théoden married Elfhild and had a son, Théodred. His youngest sister Théodwyn's children were Éomer and Éowyn.
During the War of the Ring, he fell under the sway of his advisor Gríma, called by others Wormtongue and secretly in the service of the traitor Saruman. Gandalf helped him to see the light, and strengthened, he played a major part in the war. He led his people in the Battle of the Hornburg, and in the Battle of the Pelennor Fields, where he was killed. He was succeeded by his nephew Éomer.
Relations between Théoden and his nephew Éomer and niece Éowyn were notable. Each, in loving loyalty to their uncle and king, disobeyed him in ways which proved crucial to victory in the war. Éomer let Aragorn live, when blind obedience to the king's order would have forced Éomer to kill him along with the elf and dwarf who were his companions. Eowyn left Rohan in man's garb, riding to war with Merry, and together they defeated the head Ringwraith.