Deis, along with her "sister" Myria, are extra-dimensional beings summoned to the world of humans to serve as gods. They were summoned by desert dwellers, dying in the parched lands and unable to spread beyond the oases, who wished to be able to advance as well as survive. Over the centuries after their summoning, Deis and Myria settled into the roles that had rather been thrust upon them. Deis wanted to let humans advance through hard work and achievement, but Myria disagreed. They were already in the middle of a desert and humans had struggled this long. Weren't they summoned to make life easier for humans, she countered?
Over Deis's objections, Myria soon built an orbital station with the technology from the two goddess's world, and gave her people a sprawling, advanced machine city where no one had to work. She had given her people a utopia. But, as Deis cautioned Myria they would do, her people squandered it. They began to fight amongst themselves for the blessings of Myria rather than work together. And so the city, Caer Xhan, fell into disrepair and the desert swallowed it up.
After this, Myria took what few humans survived and brought them to the other side of the world, where life overflowed. But on this side of the world was Yggdrasil. He too was an Endless, summoned to keep the world green and protect it from the encroaching desert from the other side. He took the form of a massive tree. However, he refused to allow Myria to take over the green part of the world. Deis cautioned Myria to leave Yggdrasil be. But Myria ignored her.
When Myria made her desire to take control at all costs known, Yggdrasil did not fight her, fearing for the world if their power were to be unleashed. And so she separated Yggdrasil's essence into six trees, placing five all around the green world and one on the barren side of the world. This kept him forever unable to consolidate his power again.
With Yggdrasil out of the way, Myria created a huge ocean, later called the Outer Sea, to separate the desert from the green part of the world. Then she wiped the memories of the remaining humans from the barren side of the world and placed them into the green world to live. This too was done over Deis's objections -- if they didn't remember their mistakes, she said, they would repeat them.
With Deis always criticizing Myria's actions, and Myria unwilling to listen to her sister goddess, a rift began to grow between the two. This eventually led to Deis leaving the orbital station. Deis made herself a lair in Mt. Zublo, an active volcano, and from there she watched Myria's vision for the world take shape.
A handful of people Myria had once granted wishes to far back had not disappeared from the world, though. They used the power granted to them to become Dragons, and their descendents, the Brood, still existed. Convinced that humans would destroy her now tiny world if they were allowed to keep this power, Myria decided to destroy the Brood. She chose four of her most loyal Urkan warriors and embued them with draconic powers. Then she sent them on a crusade to destroy the Dragons.
By and large Deis had stayed out of her sister's business; these were her mistakes to make. But when Myria's warriors began to destroy the Brood, she emerged from her lair and tried to speak reason. She tried to explain to the Guardians, Myria's dragon warriors, that they were doing wrong, but they took her words as blasphemy. And then they used their own powers to seal Deis in an underground room in Angel Tower. There she slept away the rest of the Great War against the Brood. Much as Yggdrasil had done, the Dragons didn't fight back, for fear that the world would be harmed. And so the Brood were mostly destroyed.
Five hundred years passed, and the Brood were mostly forgotten. In that time, history was rewritten by the Urkan people, and the Brood were cast as devils that tried to destroy the world, but were vanquished through the might of the Guardians. Deis was cast as an evil witch that had assisted the Brood, but she too had been vanquished by Myria's holy power.
But not all of the Brood had been destroyed. Some of the Brood had given up their powers and fled to the barren side of the world. Those who kept their powers conceived children and went into a deep sleep underground. Some of these children were destroyed, but one survived. His name was Ryu. he went on a journey to discover the truth about his people, the Brood, and what had really happened to them.
Along the way, Ryu awakened Deis. Deis was shocked to see Guardian Garr, and struck him several times before she found out that Garr was /helping/ Ryu rather than trying to kill him, so she decided to help. Now freed, she went back to her lair at Mt. Zublo, where a slab of stone with ancient inscriptions had been erected. There she waited for Ryu, and when he arrived, gave him the ability to sense Myria's general location. Before she was imprisoned she would have been able to take them right to where Myria was, she said, but this would have to do for now.
Deis continued to serve as occasional advisor and teacher to Ryu throughout his journey. Deis didn't show herself again until Ryu and his friends had defeated Myria. As she descended into the bowels of her orbital station, myria called out for guidance. And Deis appeared before her. She said again that Myria had gone too far with things in her attempt to destroy the Brood. But that she was sure humans would be able to figure things out themselves. That said, Deis took her place beside her sister as they descended.
However, something separated them. Deis wasn't sure what it was, but she was suddenly in another place. A city that had all kinds of people in it! Not just people, either. She had ended up in Twisted somehow. And she had no idea if her sister had come here as well...