After Korra temporarily destroyed Vaatu, Raava know that Vaatu will be reborn again inside them. Knowing this Raava thought of a plan that will might help the next Avatar after Korra's passing.
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Years have passed after Korra opened the spirit world, because of this many spirit good or bad have been causing trouble to the physical world. The chaos that it brings grows everyday and Vaatu who is still being reborn grow stronger everyday.
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We see Korra's weaken and dying body after trying to control the chaos the spirits and humans doing for trying to destroying each other.
In her passing she tapped into her Avatar State and there she see Raava weaken with Vaatu trying to take control of the connection of Korra and Raava, they fought but Vaatu is stronger then the two. In the last ditch of effort to win Raava carry out the plan and tell Korra about it, it was dangerous but if done correctly the new Avatar will be free from Vaatu's grasp.
What happened next was a gamble, Korra with all her strength tried to wrestle Vaatu clinging to them and Raava did the unthinkable and sever the connection they have with Korra and disappear to the physical world while Korra tried to absorb Vaatu making her Vaatu's vessel and jail as long as she's alive.
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Raava weaken from the fight and lost of connection to Korra tried to find a new vessel that will be the new Avatar of the world who will bring new balance on both worlds.
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Years have passed after the incident and the new Avatar is learning to bend elements as they were being taught by the members of White Lotus. The new Avatar is still unknown to the world as Korra is still alive and well.
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What they don't know is Korra is now a prisoner in her own body as Vaatu is using her status as the Avatar to create a plan with the evil/chaotic spirits to take over the physical and spiritual worlds.