In my campaign there is a level 14 illusion wizard who has used the Illusory Reality feature and silent image to create a real mind-controlled tarrasque.
Then he used Malleable Illusions to make it into a gem that he would later one-shot our big bad with.
This powergaming is ruining my campaign! Is this legal or am I missing something? If it's legal, what do I do to fix my game?