Sorry, for the bad quality, I'll make a better one soon. Ok, you've gathered those 3 crystals in the cave? The ones I've marked on the map before? They look like moonstones, just place them into the sacrifice bowl by drag&drop from your inventory. A secret portal will open. Click on it and then proceed. I'd save if I were you, why? You have to click on that tombstone there and fight the guardian. As soon as he's dead, an opening will appear in the south part of his room - proceed. The numerous bats in there are the only enemies.