Silverpate was a pirate who plundered the seas of Serpent Isle as captain of the Black Gull. After his retirement, he built the Inn of the Sleeping Bull. He used a part of his treasure for the building. He also hired a mage (reportedly Erstam) to do something to the inn, but no one knows what. Silverpate vanished after this was done.
According to the mages of Moonshade, Silverpate planned to trick Erstam so he wouldn't have to pay the mage's fee, but Erstam caught him and trapped him in a pit of daemons; hence why Silverpate was never seen again.
He was mentioned in Ultima VII Part Two.