This comes right at the end of Act 2, after you have given all six golden items to the dragon and it has threatened to kill everything.
1. Go to Nanny Ogg at the Market Square and get the carpet and the custard book (any walkthrough will tell you how, or you will just know it anyway if you've played it before).
2. Go to the Library and back in time through L-Space.
3. Take the dragon summoning book from the shelf.
4. Now, in order to advance in the game you are supposed to swap the covers of the dragon and custard books, and then replace the custard book on the shelf. But instead, put both books in your inventory and go back through the L-Space portal. You will appear in Death's Domain, where he is having a card game with the other three horsemen of the Apocalypse, in a scene highly reminiscent of the book "The Light Fantastic." Once you are delivered back to the Library you can go in again and get a new dialogue entirely.
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