After the Rivough clan attacked the free city of Darna in the Yied Desert, Mananan set off in an attempt to resolve the situation. He led an army to defeat Rivough and slayed the Rivough ruler (who might have been one of his relatives, plus was 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 Isaacian-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 Ulster possess.
Mananan's name derives from the Irish mythological character Manannan mac Lir, the god of the sea and weather.