Leitrum was Emperor of the Cath Bruig Empire during the events of Myth 3 the Wolf Age. He was respected as a great warrior and was revered by his people and allies. Leitrum was a good friend of Connacht, Damas, Ravanna, and Mydred. Mjarin was his advisor, and that would prove to be his undoing.
Leitrum put up a good fight against Moagim, but Moagim eventually struck him down dead. Curious, but given that many during that time were raised as Fallen Lords--that Leitrum isn't among them. If Leitrum was raised, he would most likely be a ghost lord under the command of the fallen. It also should be noted that Leitrum bore the Ibis Crown, a powerful artifact in the Myth world. Why he didn't use the powers of this artifact against Moagim reborn is unknown.
It should be noted that even though Leitrum was loved by his people, he was not necessarily loved by the rest of the world. Leitrum's empire began to clash with Fir'Bolg, and the Skrael are recorded as having no interaction with Llancarfan during Leitrum's reign. Many of these issues were probably due to Mjarin's actions which the forces of Light had no control over.