He hid Marth and his surviving Altean knights in Talys after his flight from Altea. He also gave Marth comforting words during this time, urging the prince to remain in hiding and play his part in the War of Shadows only when he was strong enough.
When an army of Galder Pirates led by Gazzak attacked the castle, King Mostyn was rescued by Marth and his knights. Afterwards, he donated some of his greatest soldiers, led by Captain Ogma, and wished Marth luck on his journey.
- Initially, he was not mentioned by name in any of the games, with his name coming only from supplementary materials.
- Though his name was long established, it wasn't until Fire Emblem Warriors that his name was actually used in a game, with the DS remake Fire Emblem: Shadow Dragon reffering to him as Talys king.