Hadrathus is one of the core heroes in the Conan board game, acting as your archetypal wizard. The character appears in The Hour of the Dragon, and aids Conan as he tries to regain the throne of Aquilonia. He is a high priest of the god Asura, and his church had traditionally been suspected of cannibalism and persecuted by the official state religion, the church of Mitra.
However, Conan acknowledges many gods and immediately grants freedom of religion upon taking the throne. This gains him the loyalty of all of the followers of Asura, who are instrumental in returning the Cimmerian usurper to the throne.
Frankly, Hadrathus isn't much of a magic user in the story, but true wizards are rare in Conan. They're mostly priests of esoteric gods that are privy to lost knowledge, allowing them to trick others into believing they have magical powers. Of these, only Hadrathus and Pelias (a true wizard) qualify as allies of Conan, so I can see why they used Hadrathus for this role.