I watched an anime a long time ago. The story goes like this. 4 children (2b & 2g) does their rite of passage. After that they will be able to use magic. 1girl once saves a goblin like creature from drowning. After a while in the storyline the goblin along with other goblins starts revolting against the magic people. They kill a lot of magician. And the protagonist kills that goblin. But later on they finds that those goblins were actually non magic human who were transformed into goblins by magic people.