Diamondback is a card game referred to many times in the comic book Cerebus. It is a gambling game, and immensely popular within the comic. Also, the different types of cards are used throughout the series to refer to characters and their importance / role. There are a few variations of the game, with small differences in wagering.

The Deck
A single deck consists of these 15 cards. The cards are arranged from highest worth to lowest.

1 Magician

2 Priestesses

3 Queens

4 Kings

5 Priests

In games where there are more than 2 players, multiple decks are used, but any additional Magicians are removed so only 1 remains in the deck. (2 decks for 3 players, 3 decks for 4 players, etc.)

The Hands
These are the possible hands in Diamondback, followed by their worth.

Magician-Priestess - 70 pts
Priestess-Priestess - 65 pts
Magician-Queen - 60 pts
Magician-King - 55 pts
Priestess-King - 50 pts
Priestess-Queen - 45 pts
Priestess-Priest - 40 pts
Queen-King - 35 pts
Magician-Priest - 30 pts
Queen-Queen - 25 pts
King-King - 20 pts
Queen-Priest - 15 pts
King-Priest - 10 pts
Priest-Priest - 5 pts

The Game
I've made the order of play into a numbered list to ward of confusion.

1. Each player draws a card. The player with the highest card becomes the dealer.
2. The dealer deals two card, face down, to each player.
3. Each player may examine their cards, then the dealer makes a bet which must be matched by each player. Note: there is no ante in this version of diamondback.
4. All remaining players (those who have matched the dealer's first bet) then reveals one of their cards.
5. The dealer then initiates a new round of betting. Each player must choose whether to match the bet or to fold. Only the dealer may raise the bet.
6. This continues until all the players have folded, in which case the dealer wins, or until the dealer has stopped the betting, in which case the second card of all remaining players is revealed.
7. After the second card is revealed, best hand wins. In cases where two people have the same "best hand", the pot is carried over to the next hand. If there are three or four players, and two players have the same "best hand", the pot is divided among those players. If ALL three or four players have the same hand, the pot is carried over to the next hand.
8. Repeat

Variations:
Match Diamondback: A game continues until both players, or a majority of players, agrees to end it. If a person leaves before a game is officially ended, he/she forfeits all winnings into the pot.
Dealer's Diamondback: The dealer may set an ante before dealing the cards. Also, the players must double the dealer's bets, or fold.

There is an Official Cerebus Diamondback released by Aardvark Vanaheim. If you really want to impress all the friends that come over for thursday night diamondback you'll need one.
Altogether a great game. Plus the added fun of the possibility of losing a lot of money.

Sources: Swords of Cerebus #1
Cerebus: World Tour
Dave Sim - The Usenet Interview (http://www-personal.engin.umich.edu/~hellpop/Sim.html)