The king, Theoden Ednew, was long put under the power of Saruman, who mostly worked his evil through the lies of Grima Wormtongue. When healed by Gandalf he arose and marshalled his troops to war. Through the defeat of Saruman he came, to die with honor on the Fields of the Pelennor.
References: Unfinished Tales, Lord of the Rings, Rotk's Appendix A, "The
Kings of the Mark: Second Line",
Tolkien Quiz Book
King Theoden was the son of King
Theoden was succeeded by Eomer, Theoden's sister's son, since Theoden's only son, Theodred, died before Theoden did.
"Theoden". Artist Timos Kouimtzoglou. Rolozo Tolkien site
Image: "Theoden espies the Serpent Banner". Artist Michael Kaluta. Rolozo Tolkien