Roughly nine months prior to the events of the Silver Seed, Andriss and a band of warriors and mages set out to reclaim the Abandoned Outpost and install many trap mechanisms. They indeed set the traps, but then were ambushed by Chaos forces. Ardiniss' forces were slaughtered and he himself found his death near the torture chamber of the outpost.
The Avatar found his body in Ultima VII Part Two. He had an interesting scroll, which helped to deactivate a force field blocking the way to the Belt of Strength. Afterwards, the Avatar returned the body to the castle to Surok, who had asked for it.