During the War of Heroes, Blaiddyd was one of the Ten Elites who fought alongside Seiros, the Four Saints, and the Adrestian Empire against the forces of Nemesis. Blaiddyd bore the Crest of Blaiddyd, a blessing from the Goddess, and wielded the legendary weapon Areadbhar. After the war, Blaiddyd founded House Blaiddyd in the northern region of Faerghus. A branch of the Blaiddyd royal family intermarried with House Riegan after the founding of the Holy Kingdom of Faerghus.
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In truth, Blaiddyd 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. Blaiddyd's descendants became nobility of the Empire and later, the Holy Kingdom of Faerghus, passing down his Crest through the centuries.
In Imperial Year 1185, if Claude was sided with, a warrior named Blaiddyd 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 Blaiddyd 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. Blaiddyd, the other Elites, and Nemesis were ultimately defeated, thwarting the final revenge of Those Who Slither in the Dark against the people of Fódlan.
Blaiddyd is the moniker of a legendary Brittonic king who encouraged various necromantic practices, such as communing with and raising the dead. The name roughly translates to "Wolf-lord" in Welsh.