Text (SH+errata): At the beginning of each player's upkeep, that player discards a card at random. [Oracle 2005/08/01]
The ability is controlled by the player who controls Bottomless Pit. This means that Bottomless Pit can trigger Guerrilla Tactics and other abilities which trigger off an opponent forcing you to discard. [WotC Rules Team 1998/05/01]
