King Constantine was a Celtic ruler. A group of Gallifreyan Interventionists helped his tribe escape the Romans by creating an other-dimensional space. This realm, Avalon, was maintained by Constantine's sleeping mind and technology provided by the Interventionists. By the time the Eighth Doctor was brought there in the early 21st century, it had existed for two thousand years.
He dreamed an alternate self into existence on Earth called Constantine King, or Rex. Gandar managed to hypnotise Rex and command him to kill Compassion. To prevent this, the Doctor removed him from the equipment and woke Constantine, which began to destabilise Avalon. However, Queen Mab was able to use her amulet to return the king to sleep, saving the realm. (PROSE: The Shadows of Avalon)