Wolfgang

The father of Mondain, Wolfgang, was a much-respected king and a busy mage. The strange and violent actions of his second-born son greatly disturbed him. When Mondain was fifteen years old, Wolfgang told him he was to be sent away to live with "the brothers in the abbey," to learn to be more humble and compassionate. On Mondain's return as a better person, his father would give him his prized ruby gem, which magically harnessed the sun's power.

Mondain killed his father the next night and took the gem for himself, using its own power against itself to render him immortal. Thus, Wolfgang's son ushered in the first Age of Darkness in Sosaria, as chronicled in Akalabeth and Ultima I.

Trivia

 * To date, nothing is known of Mondain's mysterious sibling - Wolfgang's first-born...