Duke Nemesis the Inescapable was a Chthonian demon who led the first of twelve demonic legions that Hostilius the Autocrat had given to the god Zaros. He is also the author of the Codex Ultimatus, which can be obtained after The Dig Site.
At some point in the Second Age, he led 100 avernic demons - a small portion of his legion - to Gielinor, and was in charge of scouting the area where they emerged. Leading his army, he learned of a local hydra god known as Loarnab, and after Zaros arrived, they lured it out and sent his troops to attack it at Zaros's command.
After an hour of combat with the beast, nearly all of Nemesis's troops had been killed, and Zaros ordered the remaining troops to retreat. Zaros then entrapped Loarnab using his complex Magic in order to use it as a power source to create a portal large enough to bring the rest of the demonic legions from Infernus to Gielinor, with the rest of Nemesis's legion being the first to emerge.
Nemesis was initially hesitant due to the cruelty Zaros was showing to Loarnab for no apparent reason, not wishing such torture on even his most ruthless opponents save for one, but once what Zaros was doing become clear, he praised his brilliance.