After the Rivough clan attacked the free city of Dahna in the Aed Desert, Mananan set off in an attempt to resolve the situation. He led an army to defeat Rivough and slayed the Rivough ruler (a fellow descendant of Od, also the father of Galzus), offering said region to Grannvale in a bid to propose a truce. However, Mananan was captured by Grannvale's soldiers and executed by Reptor, resulting in the continuation of the Isaachian-Grannvaleian war. Manfroy was the mastermind behind Mananan's death, scheming with Lombard to attain more power for themselves.
The Fire Emblem Treasure's family tree reveals that Mananan may have had Mariccle and Ayra with two different anonymous women. This is also suggested by the fact that his grandson Shannan lacks the Nihil skill, which Ayra and her children Larcei and Scáthach possess.
Mananan's name derives from the Irish mythological character Manannan mac Lir, the god of the sea and weather.