Every millennia, in order to maintain the balance cosmic balance of magic across The Multiverse, a tournament is held where many great magicians battle to prove themselves as the most powerful mage there is. The contestants are taken from their homeworld and brought into another dimension and they are told that they must fight in order to return home to the world of their origin. The reward to the winner whoever comes up on top above all others are given a any wish they desire.
Eventually it turns out it was all just a ruse; the true purpose of the tournament was something far less trivial. A long time ago, researchers on an advanced world discovered that every dimension was connected to a single source, and that magical resources move back and forth between the source and the various worlds in an eternal cycle. However, whenever a major magical event occurs, the balance of magic in all dimensions will shift a bit, and when all of these little shifts pile up, larger dimension quakes will occur and, in the worst case scenario, lead to the destruction of all dimensions. To counter this, the ancient researchers devised a dimensional magic tuning system called "Kyrios", which is managed by an Artificial Human named Nowel. In addition to the management of Nowel, the maintenance of Kyrios requires the sacrifice of "Telos", a person who possesses strong control of magical abilities. The body of Telos shall rot inside Kyrios until they fall into a long sleep. The final duty of Telos is to select a new Telos. And the lucky tournament winner is the one who will become the new Telos (unless if they're ineligible due to various reasons then their friend becomes the prime candidate). It's a fate that has been repeated since far off ancient times.
Naturally, the tournament winner is quite against that plan, so after a huge Final Battle, Nowel's defeat despite the backing of Kyrios makes her realize that maybe Kyrios isn't the ultimate solution that she believes it is, and after the victor makes her realize that with all the other worlds out there, surely everyone can think of a better alternative together that won't require sacrificing anyone, the victor invites her to become friends (Or tries to get her to spill the principles of this world's magic; depending on the character).
