Source: The Infernal Syndrome, pg(s). 65
Eris Ruel was the son of Daidian Ruel, founder of the feared Chelish knightly order known as the Hellknights. He lived in the late 46th century AR, in what was then his country's capital, Westcrown.
Eris came under the influence of a demonically-inspired cult called the Path of Grace who masqueraded as devout followers of Aroden. Due to their meddling he committed suicide in Lamashan of 4575 AR by overdosing on pesh.
His death, as well as the murder by the Path of Daidian's wife Lileia, caused his father to renounce Aroden and vow to bring the cult to justice. Blocked by powerful members of Westcrown's ruling class, Daidian took matters into his own hands and gathered a group of former guardsmen and soldiers to bring this about. These vigilantes formed the nucleus of what eventually would become the Hellknights.
Obsessed with bringing his son back from Avernus, Daidian began to collect information on Hell and its inhabitants. This research (under the tutelage of many cunning devils) greatly influenced his thinking, and made its way into the organization and outlook of the Hellknights themselves, changing them forever. Eris' soul was never retrieved, however.