Once a great tree spirit, Vetel consented (back before the 3rd age) to serve as the prison for the spirit of a vicious female druid who had learned to body-jump (becoming almost immortal that way). The tree turned to stone; not really alive in the normal sense. It was then enclosed in a great cave, deep underground. Locked into the tree by wards, this malevolent spirit would be incarcerated for eternity.
All was fine, and Vetel was a kind, benevolent being, eager to grant shelter to oppressed or homeless people. The wards extended its life by drawing on the strength of other kami and the demonic spirit was quiescent. Then came the Cataclysm. In the disruption attendant to that event, the wards cracked. Slightly. Not enough for the spirit to escape, but enough for its malevolence to infect Vetel. In its weakened state, it was unable to fight off the infection, and succumbed. As its last act, it locked a curse on the imprisoning wards. If the wards failed, they would implode and take the tree (and Vetel, dependent on the tree-spirit's vitality) with it. So now, Vetel, the ancient body-jumper, had a fixed form. This it did not like. But it waited.
A few years after the Cataclysm, a band of gnomes entered the caverns looking for shelter from the disasters unfolding. They found Vetel, who struck with them a deal. They would care for the tree and the wards and worship her as their goddess, and she would protect them. This deal was false--she had not the power to do this. But they didn't know it. She chose a vessel from the young gnomish women to become her "priestess", who would be overcome by her power and act as a mobile host for a portion of her ancient spirit (and a set of sense organs, for Vetel hungered for sensation). But gnomes weren't that satisfying. Too small, too serious, not glamorous enough. And being small, the vessels burned out fast.
A generation later, the ancestors of the wind elves arrived at Pyhin, running from a bad winter storm, and Vetel saw an opportunity. She struck a similar deal with them--they would worship her and she would protect them from the storms and from outside harm. Part of this deal was that now the "vessels" would be chosen from among the wind elves. Taller, glamorous, and most importantly compatible, they'd live much longer before deteriorating and would tolerate much greater separation from the main tree.
Important note: the people do not know that she is a possessing spirit. They see the vessels as being inspired by the goddess, not being soul-eaten. And the deterioration is a closely-held secret among the caretakers (all gnomes). To most people now, the vessel is ageless and eternally young.
Girls are chosen at about age 8 every year. They are trained in her service (satisfying the vessel's urges and catering to her every whim) and are brainwashed into true devotion, willing to die for the goddess. When the vessel is deteriorating (which happens unpredictably but rapidly), those of appropriate age (post-pubescent) are brought to her and she tries to inhabit them by turns, sticking in the first one that works. Failed candidates are taken to Talvi (secretly). If a candidate reaches the age of 18 without being tried, she's released from direct service and married off (becoming a priestess to the Clans). Those who experienced a transition (but weren't tried) are often mentally broken; the others don't remember much other than "bliss".
Vetel is addicted to sensation, especially sensual. She pulls young warriors from among the Clans and uses them, draining them dry and often torturing them. Their husks are discarded to Talvi. Her servants attend her constantly, keeping her in a state of arousal. She likes inflicting pain, especially psychological pain. This breaks many of them (who are discarded). She encourages "competition" (fighting, often with bloodshed) among her faithful.
 By being worshiped, Vetel has gained tremendous power. She's searching for a way to undo the curse. At present, she (in her vessel) can only wander so far--the big risk is when the vessel goes to Talvi. That portion is separate and can be broken free of the main host (for Vetel is now a composite entity containing the spirits and memories of all those who served as her vessels) if it doesn't return. Her power is mostly over illusions and perceptions.