When Nergal required large amounts of quintessence (human life energy), he dispatched Ephidel to meet with Darin, the marquess of Laus. Ephidel was able to convince Darin into planning a rebellion against Ostia, and the deaths caused in the ensuing war would release enough quintessence to please Nergal's needs. Ephidel also visited the other lords whom had pledged themselves to aid Darin's rebellion.
Ephidel was first seen in "Chapter 12: Birds of a Feather", when he was staying at the fortress of Helman, the marquess of Santaruz. Eliwood was on his way to Santaruz to investigate Elbert's disappearance, and Helman did not think he could bring himself to lie to Eliwood, so Ephidel volunteered to drive Eliwood off without harming him. Ephidel sent his men out to kill Eliwood. When Helman found out about Ephidel's attempt to kill Eliwood and his men at the beginning of "Chapter 13: In Search of Truth", Helman threatened to tell Eliwood everything about Ephidel's operation. This led Ephidel to decide that Helman was no longer of any use, so he killed him.
Ephidel then moved to Laus Castle with Darin. Eliwood and his men soon advanced on Laus Castle in "Chapter 14: False Friends", and Darin's son Erik was dispatched to deal with them this time. But as Erik failed, Ephidel forced Darin to retreat the castle with his army, making Darin abandon both his son and his home. They then attacked Caelin Castle in "Noble Lady of Caelin" (Eliwood Chapter 15, Hector Chapter 16). When Caelin's marquess, Hausen, attempted to talk sense into Darin, Ephidel stabbed him and sent Leila to dispose of the body, unaware that Leila was an Ostian spy. Eliwood continued to fight his way through the Laus troops, so Ephidel convinced Darin to retreat once again, this time abandoning the remnants of the Laus army.
Ephidel took Darin to Nergal's base at the Dragon's Gate on Valor, the Dread Isle. Nergal sent Ephidel and Darin out to kill Eliwood and his group when they arrived on the island in "The Dread Isle" (Eliwood Chapter 18, Hector Chapter 19). Ephidel caught Leila spying on the conversation with Nergal, so he summoned Jaffar to kill her. He then had Jaffar leave her corpse in the woods to taunt Eliwood's group. Ephidel later confronted Eliwood and his comrades face-to-face for the first time on the outskirts of Dragon's Gate in "Dragon's Gate" (Eliwood Chapter 19, Hector Chapter 20). He taunted them and then teleported away with Ninian, whom Negral wanted alive for his ceremony. Ephidel then left Darin to take care of Eliwood's group while he brought Ninian to Darin.
Ephidel then conducted the ceremony with Ninian to summon a dragon. However, Nils's interference caused the dragon to crumble and explode. Ephidel called for Nergal's help, but Nergal ignored him and Ephidel was killed along with the dragon.