Cartomancer is a feat from The Book of Many Things. It allows the player to use a deck of cards as a spellcasting focus, and confers a few additional benefits. It also gives you the prestidigitation cantrip and a lets you use it subtly.
By far the most important part of the cartomancer feat is the Hidden Ace ability. This ability allows you, after a long rest, to store a magic spell in a card. The spell can then be cast from the card as a bonus action without expending a spell slot. Here’s the kicker though: you can use any spell from you class’s spell list as long as you have a spell slot for it.
The ability of the Cartomancer to use a deck as a spellcasting focus is mostly just flavor. All of the other decks that are usable as spellcasting focuses do not require this feat. Even so, with a deck like the deck of dimsions, you would be able to use your cartomancer abilities.
You could, I suppose use this feat to let you use a Deck of Many More Things as a spellcasting focus. It would be almost impossible to use the cartomancer Hidden Aces Power though. Cards drawn from the decks of things disappear as soon as they take effect, so they won’t be around for you to cast spells from later. This is, of course, not to mention the incredible risk a character would be taking each time they attempted to imbue a card with magic. How long would it take to have your soul ripped from your body, or all of your magic items destroyed?
OK, so now you have a cartomancer? Need some nasty monsters to throw at them? Check out my Hotlist!