Before Wan (Wan means soft, pleasant(kind), attractive) became the first Avatar, he had taken another male as his lover and his name was Manchu (Manchu means pure). During the Harmonic Convergence, when Wan and Raava bonded permanently, Manchu was also fused by Spiritual Energy and became the first Caster⋆. When Wan died, Manchu knew in his heart that while Wan and Raava would be reincarnated as the next Avatar, Manchu would reside in the Spirit World and forever be parted from Wan, his true love. Heart-broken, but determined, Manchu used his magic (soon after the Convergence, Manchu gave some of his magic to his brother, sister, their spouses, and their children) to make him young again and then used Blood Magic to make him immortal and accidentally became the first of the Vampires.
For the next 10, 000 years, Manchu did create other Vampires by turning other Humans, but he kept his vow to Wan by never taking another lover. When Wan and Raava were reincarnated for the first time as the second Avatar, Manchu drank some of that Avatar's blood and gave some of his own in return, therefore creating a permanent connection to the Avatar that allowed Manchu to always feel when an Avatar died and was reincarnated. From then on Manchu has been a constant companion, friend, and mentor to the Avatars.
10, 000 years later when Avatar Korra lost her connection to Raava and her past lives, the spirits of past Avatar's were finally free to resided in the Spirit World and Manchu was able to finally be reunited with his love, Wan. Ω
⋆- Casters are Humans who have a Spark that has a special connection to the Spirit World; this bond allows a Caster to draw on a limited resource of Spiritual Energy to cast into the Mortal Realm, this power came to be known as the Arcane Arts - Magic. There are Dark Casters who follow the Path of Vaatu (Warlocks are males and Witches are females), Light Casters who follow the Path of Raava (Sorcerers are males and Sorceresses are females), and Neutral Casters who keep the balance by following both Paths (both genders are known as Sages). Casters were made when Manchu started wasting away because he possessed too much Spiritual Energy and so gave some to his family allowing them to become Casters as well.
Ω- There are two alternate endings to this story: one is where Manchu finally allows himself to die by taking off his Daylight Ring and allowing the sun to burn him therefore becoming a Spirit himself to join Wan in the Spirit World, and the other is where Wan is somehow able to have a body again and allows Manchu to turn him into a Vampire so both Manchu and Wan can spend eternity together. With either ending, Manchu and Wan are rejoined again and able to spend eternity (as Spirits in the Spirit World or as Vampires) with the one they truly love - each other.