Assumption is the name of the Tarot poker game that serves as the plot axis for Tim Powers's amazing dark fantasy novel Last Call.
In the novel, the game is played every 20 years and the winner becomes the new Fisher King, a modern-day mage who can control chance and chaos and, ultimately, the fate of the world. Most of the game's thirteen high-rolling participants are unaware of the true nature of Assumption; they think they're just playing an arcane sort of high-stakes poker. The book's wealthy antagonist, Georges Leon, uses the game to capture people's bodies and souls. Those who have lost their souls in a game are usually totally unaware of it until the next 20-year game. During the following cycle, Leon evicts said souls and transfers his consciousness into their empty-but-living bodies to increase his power and influence and attempts to achieve immortality.
Leon is a modern-day Cronus who invented the game as a way of spiritually creating children he could then turn into puppets. In the first part of the book, he is shown to be possessing his own young sons and evicting their souls, but while he is in the process of taking over his youngest son Scotty, the boys' mother enters the room to rescue her son. In the melee, she shoots Leon in the groin, castrating him. In order to maintain his power, he must find a way to create more children though he is biologically incapable of doing so.
The game of Assumption results in the creation of a winning hand created from two different player's initial hands that spiritually represents the fruit of the player's souls; instead of being "lost" it is actually "sold" by one of the hand's spiritual parents to the other. The "winning" player who takes the pot for the hand has in effect sold his or her own soul to the other "parent" of the hand, and the taker of the money then assumes the spiritual role of the child of the other player ... and is thus ripe for possession.
Assumption must never be played over "untamed" water like a natural lake, river, or ocean. Man-made bodies of water like Lake Mead are useful sites for play, and in fact the climactic final game takes place over that lake.
For details on actually playing the game, read Using Tarot Cards For Games.