Star Ember Academy: the Boy Chosen by the Taboo

The young protagonist was originally an ordinary watchmaker's apprentice in a border town, but was selected by an out-of-control ancient magic on a "starry night" and entered the "Star Ember Academy", the highest magic school in the mainland. He found that the "Thirteenth Source Spirit" that had been erased by history was sleeping in his body, and the academy, the kingdom, the Holy See, and the believers of the old gods were all vying for him.