Prologue
In ancient times, the lion turtles were the guardians of humans, protecting them from the vicious spirits that roamed the wilds around them. They granted humans the gift of control over the element they carried, whether it was water, fire, earth, or air. Things were peaceful enough, yet they were not fair within the lion turtle cities. One human decided to change that.
His name was Wan, a scrawny boy with no money and barely enough food for himself and his companions. His actions led him down a dark and dangerous path. Kicked out of his lion turtle's city, Wan roamed the spirit wilds, desperately seeking shelter and a new way of survival. For some time, he feared his decisions would be his downfall.
Eventually, Wan was able to entrust a spirit friend that allowed him access to his oasis after he saved an innocent cat deer. This spirit told him of other lion turtle cities, and Wan began a journey to find the other humans.
Along the way, Wan unknowingly broke apart the spirits of light and dark, Raava and Vaatu, and released evil into the world. Fearing all was lost, Raava fled to find Vaatu again, praying she would be able to ensnare him before the great harmonic convergence.
Wan continued his journey and discovered another lion turtle city. The humans dwelling there were granted the gift of control over air, and they lived harmoniously with spirits. Seeing this new way of life, Wan wished to show his fellow humans of the fire lion turtle's city that they could live together with spirits.
Before he was able to leave, Vaatu appeared. He turned the spirits dark and threatened 10,000 years of chaos after harmonic convergence. Raava, in her newly weakened state, was not able to fight off Vaatu. She feared that she would lose the upcoming battle. However, there was hope.
Listening to the lion turtle's advice, Raava and Wan decided to work together, she holding the gift of the elements while Wan used one, then bonding with him for a moment to change elements. They sought out the other lion turtles of water and earth.
When harmonic convergence was upon them, Raava and Wan went through the spirit portals to fight Vaatu. They bonded for eternity in the spiritual energy of harmonic convergence, giving Wan the power to control all of the elements, officially creating the avatar and defeating Vaatu.
10,000 years later, a new avatar named Korra discovered again the spirits Raava and Vaatu, fighting in an epic battle of light and dark, successfully defeating Vaatu to restore another era of peace.
That is what she believed, anyway.
Korra faced other threats after her fight with Vaatu and the dark avatar Unalaq. A group called the Red Lotus threatened to overthrow all leadership. Led by Zaheer, they wanted to use the avatar to take down world leaders and engulf the world in chaos. Korra believed she defeated them, but all was not as it seemed.
It is true that Zaheer was captured, P'Li was killed, and that the Red Lotus returned to hiding, but their threat to the world was as strong as ever.
As Korra was physically and emotionally scarred by the venom of the Red Lotus, and later distracted by the Red Lotus' tyrannical Earth Empire leader, Kuvira, they plotted to bring about her demise. However, the final steps weren't put into place until they were sure where the next avatar would be born.