Seth was an immortal.
In the 24th century, he was known as the politician Varley Gabriel. He organised a colonisation mission, but it was a cover to get the best people to use for a project on another planet. The remaining support colonists he planned to sell to the Profiteer of Ephte as slaves.
The TARDIS landed on the colony ship Mayflower on the way to Arcadia. The Sixth Doctor stopped the Profiteers from taking the colonists. Seth swore revenge, but the Doctor ignored him, reprogramming the ship so it would go to Arcadia. (COMIC: Profits of Doom!)