On this island is a lighthouse, rotting from age and sea water, which is never lit. There is nothing inside it, save for a spiraling staircase that leads to the top, and an ancient, dusty bookcase.
The case is filled with unmarked books, bound in ancient leather, save for a single space. If you remove a book from the shelf, it will fling itself open in your hands, and the words inscribed in it shall start screaming to the air.
You must wrestle the book closed and shove it back on the shelf, or the immortal evil contained within its pages shall break free, and you will be forced to take its place, with pages, ink and binding crafted from your own flesh and blood.
However, if you bring the correct book to the island, and place it in the empty space, the lighthouse will light. As long as it is lit, the world shall enjoy an unending paradise, for all the evil in the world will be contained in the lighthouse. And while it is lit, nothing can go in or out.
The only problem is that you will be trapped for eternity in there with all the evil ever known or conceived, by man or god.
The only way to escape... is to douse the light.