During the War of Heroes, Riegan was one of the Ten Elites who fought alongside Seiros, the Four Saints, and the Adrestian Empire against the forces of Nemesis. Riegan bore the Crest of Riegan, a blessing from the Goddess, and wielded the legendary Failnaught and Sword of Begalta. After the war, Riegan founded House Riegan in the eastern region of Leicester. His descendants eventually intermarried with the progeny of King Loog after the formation of the Holy Kingdom of Faerghus.
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In truth, Riegan and the other Ten Elites were allies of Nemesis, not Seiros. After Seiros prevailed over Nemesis and the Adrestian Empire completed its conquest of Fódlan, the Church of Seiros revised history. Riegan's descendants became nobility of the Empire and later, the Leicester Alliance, passing down his Crest through the centuries.
In Imperial Year 1185, if Claude was sided with, a warrior named Riegan served as one of the ten commanders of the army of Nemesis, who had been revived by the technology of Those Who Slither in the Dark. While Claude dismissed the idea that this Riegan could have been the revived original, the fact that Nemesis himself was revived lends credence to the possibility that Those Who Slither in the Dark truly did preserve the Ten Elites and restore them to life. Riegan, the other Elites, and Nemesis were ultimately defeated, thwarting the final revenge of Those Who Slither in the Dark against the people of Fódlan.