The Relic and the Tilted Stone

You have found your way to the Chapel of the Sunken Order, hidden inside a sea-stack on the island's eastern shore. The tide will seal the entrance in an hour, but you are not thinking about the tide. You are thinking about the stone altar at the centre of the chapel — and the heavy iron reliquary that sits, very slightly, off its plinth. Someone has moved it. And the plinth, you notice, is not a plinth at all.