Nergal is the main antagonist of Fire Emblem: Rekka no Ken, and is the only character in the series who belongs to the Dark Druid class. He is thousands of years old, wise beyond belief, and wields the strongest dark magic tome in his specific game game, Ereshkigal. He is responsible for the golden-eyed morphs (i.e. Fire Emblem, Limstella, Ephidel) that travel the world looking for the quintessence of powerful individuals to give him even greater strength to call dragons through Dragon's Gate where he holds Ninian, Nils, and Eliwood's father early in the story. He stirs war in Lycia by controlling an assassin group called the Black Fang through his morph Sonia.
Nergal was the one who called Ninian and Nils through Dragon's Gate a few hundred years after he and Athos discover the dragon world of Arcadia where dragons and humans lived peacefully. In the last chapter of the game, Nergal finally uncovers his face from the turban that hides it to reveal a wound along his forehead and his right eye that he says Athos is responsible for. After Ninian is accidentally killed by Durandal through Eliwood, it is her quintessence that actually opens Dragon's Gate from which he eventually calls three fire dragons through before dying, but after being defeated.
If the player clears several side-quest chapters of Hector's campaign, it is revealed that Nergal was in fact the father of Ninian and Nils through a relationship with a dragon, and sent them through Dragon's Gate to protect them when his wife was kidnapped. Afterwards, he turned to dark magic to make himself stronger so he could open the Gate himself and reclaim his children, but lost himself to the darkness and forgot why he ever wanted to open the gate in the first place, only remembering that he needed power and that the Gate must be opened.