King Tommen I Baratheon was the younger brother of King Joffrey and Princess Myrcella. Though legally the son of the late King Robert Baratheon and Queen Cersei Lannister, his true father is Ser Jaime Lannister, the Queen's twin brother. His sole biological grandparents, Tywin and Joanna Lannister, were also first cousins.
After his brother's death during his wedding to Margaery Tyrell, Tommen assumed the throne under the name of Tommen of the House Baratheon, the First of His Name.After witnessing the destruction of the Great Sept of Baelorand learning of the death of his beloved Queen among all the casualites, Tommen committed suicide by jumping out of a window in the Red Keep.
(source GOT wiki)