Diarmuid was the King of Tyr. A wise and just ruler during the Great War. He fought long and hard against the coming dark. Diarmuid left a massive legacy which his heir to the throne (Cu Chulain) took up after Diarmuid's death.
Diarmuid fought with two blades or he would fight with two spears. When he faced the Watcher it was with his two swords. He fell in battle wielding the two magical blades that turned the undead into ash.