Secret Cave glitched

[Phoenyx]

I beat the game, getting all 5 crystals in each zone, but the secret cave is still closed. I went back to all 3 zones to make sure, and they all say I have 1 crystal, so presumably my memory is correct and I did get all 5 each. I also don't have the Prophecy achievement, but I'm not sure if there's any puzzle(s) in the secret cave.

[Bilgot the Heartless]

This thread is from one month ago and I don't know if you still need help but to open secret cave you must put all remaning crystals into altars where seals were. So you need to go back to every pyramid.

There is a one puzzle in that cave, when you finish that puzzle you will get Prophecy achievement.