I think the issue is that the ritual spell never showed up in my spell list even though the game told me I learned it. Either way, I finished the game last week and it was a lot of fun!
Wait, did you equip the ritual gemstone? The steps are as follow: you find the magical gemstone in the first temple, find the secret room in the second temple to turn it into a ritual gemstone and then you equip the new stone when you want to use the spell.
