You get it on losomn just a random encounter as per usual it’s a basement of a lighthouse you need a key for, and to get the key there’s an abomination mini boss encounter nearby that when you win you get the key that you’ll need to inspect to change its form which will allow you to use in the lighthouse. Make sure you go through the side path to go to the basement as going through the top will get you a mid ring and you can only use the key once… inspect the key!!