The dimensional universe known as the Other Side of the World was created in a different way from any other known universe. There was no big bang, but all was created in a steady state flow of primal matter, resulting in a place where time - as we know it - doesn't exist. There, nothing ages or dies; and if something s destroyed it is immediately replaced.
The being known as the Warlock of Ys is the only known native of that universe. In time he became bored with his world and, with the aid of his magic mirror, saw Earth. He longed to go there in order to experience something new and perhaps to conquer the world, but even his mighty powers could not enable him to leave his own world.
However, he managed to make contact with the Senes - priestesses of the god Teulates, in what is now Brittany. At his direction, they constructed the Gate of the Blue Flame, through which one may enter the Other Side of the World. In the time of Julius Caesar, a Roman Centurion was the first known to have stumbled through this gateway and become lost in the timeless dimension.
By the late 5th Century, good king Gradion came to the throne of Comouaille; he was a devout and Godly man, but his daughter Dahut was evil, delighting in her sinful ways. Even so, her father adored her and gave her anything she desired. At her request he built the great city of Ys in the bay of Douarmenez, with a great dike to keep the sea out. The great bronze doors could only be opened by a silver key which Gradion wore on a Golden chain around his neck.
Dahut took Lucifer himself as her lover, and he demanded that as proof of her love, she provided him with the key to the dike. She stole it and Lucifer used it to open the gates and at flood tide and drown the city. Only Gradion escaped. Dahut became a Siren, luring men to their deaths. Because the Gate of the Blue Flame was near to the site of Ys, it was said to be connected to it, and from it the Warlock took its name.
When Zatanna was searching for her father Zatara, she reached the Other Side of the World and found the Warlock sending a magical creation to lure Green Lantern (Hal Jordan) there, or he believed only the power ring could enable him to enter earth's universe.
Working together, Zatanna and GL defeated the Warlock, leaving him alive but motionless and stripped of his magical powers. Yet he had worked one last spell, causing his and Hal Jordan's life energies to slowly transfer, so that a few years later they changed places. GL minus his ring was motionless in the Warlock's tower, while the Warlock was posing as Green Lantern. Zatanna was placed under a spell to keep her from warning anyone else what had happened.
However, she managed to alert some members of the JLA and they tracked down The Warlock and freed Green Lantern. The real Green Lantern and the Warlock then exchanged places and the Warlock was powerless once more.