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 2012, it had existed for two thousand years. Gandar managed to hypnotise Constantine, by whispering into his sleeping ear, 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)
Constantine King[]
He dreamed an alternate self into existence on Earth called Constantine King, or Rex. (PROSE: The Shadows of Avalon)
Brigada[]
The presence Compassion's transformation into the Type 103 TARDIS caused Constantine/Avalon to create a benevolent, interventionist "goddess" within the Time Vortex, who began to infleunce Avalon even before the point of her "birth" (Compassion's transformation and the end of the War in Avalon). The Knights of Brigida claimed to follow her, and one of them, Joe Boyce, convinced the Brigadier to move on from the suicidal thoughts induced by his intense grief for his dead wife.(PROSE: The Shadows of Avalon)