Introduction to Tigris & Euphrates
As a player, you are in control of four leaders: king, priest, farmer, and merchant. These leaders are represented by round discs of black (king), red (priest), blue (farmer), and green (merchant). Leaders are placed on the board to form kingdoms. As tiles are added to a kingdom, whoever controls the like-colored leader in that kingdom gains prestige (in the form of Victory Points) as well as increased political strength.
As kingdoms merge (external conflict), conflict almost always ensues. Conflict occurs if the two merged kingdoms have the same leader type (i.e. king and king). To resolve the conflict the leaders look both to their support in the kingdom as well as support from the player's hidden pool of tiles. Once support is declared, the victor is determined, and all of the loser's supporters as well as the leader are removed from the board.
As if the life of a leader weren't uncertain enough, they can also become victims of coup attempts. A coup (or internal conflict) is started by placing a leader in a kingdom that already has a leader of that color. Unlike an external conflict, an internal is resolved based on religious support. Whichever leader survives the coup attempt remains on the board. The other leader is removed.
-- contributed by FullTinCan