Ouranus was a terrible husband and father to Gaea and the Titans but no one ever knew that while Gaea did not entirely hold his affection, his love and loyalty lies to another goddess and I mean, another being way out of his league and out of his world, the personification of Light, Raava, the Mother of Creation of the reflection of Earth.
Both of them rule different worlds and both had responsibilities. While in Ouranus and Gaea's world they birthed their offspring, Raava and her brother created their own.
However, the strangest things are, instead of birthing their offspring, Raava and Terranus had children through their emotions. From them sprung forth the lion-turtles who held power over the elements. They had a lion head with turtle bodies that was the size of an island. From Raava's passion and love and her brother's hate and hunger for power was the lion-turtle with the element of fire. From Raava's kindness and warmth and her brother's twisted manipulative nature was the lion-turtle with the element of water. From Raava's protective and loyalty and her Terranus' stubborn and aggressiveness was the lion-turtle with the element of the earth. And lastly, from Raava's peaceful and calm demeanor and her brother's mischievous and fleeting nature was the lion-turtle with the element of the air.
The water lion-turtle roamed the vast ocean and appeared to be a floating island. The fire lion-turtle settled inside the biggest volcano on earth and appeared like it was bathing in the lava with an island on its back. The air lion-turtle floated up the skies like a huge sky island above the clouds. The earth lion-turtle buried itself in the endless sand in the desert only revealing its shell and always mistaken for a jungle paradise in the middle of the desert.
Ever since Raava and Terranus, also known as Yin and Yang, came to life, they had been always battling each other for two different reasons. For Raava, it was to preserve life and for Terranus, it was to kill his sister and dominate the world with darkness. Divine blood was shed from their eternal battle thus creating the lesser spirits who roamed the earth.
The lion-turtles, too, had created beings to inhabit their shells surface; they were called the humans. The four lion-turtles steered clear of the main land wherein the spirits lived for not all spirits are good. But when people on the lion-turtles grew in numbers, their food supply decreased until they are forced to fight each other. The violence upset the lion-turtles so they decided to introduce the mainland's resources to the people. That's when the problem began.
The spirits were disturbed by foolish humans, who destroyed nature in the process of taking food; the spirit punished them by either killing them or turning them into trees, rocks, etc. This forced the lion-turtles to bless the people, who ventured to the mainland, the power of their respective elements so they can fend for themselves but there were power hungry humans who wanted to take the mainland. They rallied the others and used the elements bestowed unto them to kill some of the spirits.
A foolish, ignorant and easy going human was chased off by spirits and he found Raava and Terranus in a heated battle. Terranus tricked him into helping him get away from Raava and once Terranus was freed he turned the spirits dark to destroy Raava. As more and more spirits turned dark, humans gave in to terror and Raava started to grow weaker each day. The human, Wan, regretful of his actions; he communed with Raava to merge his soul with hers to lend his life force to help her defeat Terranus. They succeeded to lock Terranus in a cage but the act had cost Wan's life. Raava decided to separate the Spirits from the humans so she locked them in a different dimension. In remembrance of her mergence with Wan, Raava decided that the balancer between the Spirit world and the Human world will be chosen from Wan's descendants. The balancer, the most powerful person in their world, was soon called the Avatar.
As years passed, the Mainland became the home of humans and the lion-turtles went into hiding as ordered by Raava. The Mainland was divided into four countries; the Water Southern and Northern Tribe, the Earth Kingdom, the Fire Nation, and the Air Nomads.
Raava didn't take any chances with the humans as she only trusted her host, the Avatar, so she chose one person of pure heart from each country and turned them her priests. These priests are blest with immortality and were given a task create the Tekhis and protect it forever. The Tekhis has the written spell for the divine prison, Armadethys, wherein Terranus was held. The priests are given the order to only use it when something happens to Raava. The priest hoped that they would never have to use the Tekhis. What they didn't expect was that the Tekhis itself would be the reason of sending their world into destruction.