Book of Demons

You'll find a sufficiently high quality card that you need Deckard Cain The Sage to identify it after you've delved deep enough into the ruins. It's random, but I got mine on the fourth or fifth adventure, just before the first true boss. The achievement does not unlock upon pickup, you do have to return to town, speak with The Sage, and pay him a nominal fee to identify the card(s). Then the achievement pops.