Final answer:
The BOMB BAG is the item that drops a bomb pickup every 2 rooms, unlocked by beating Satan with Cain in the game. It provides bombs essential for combat and exploration.
Step-by-step explanation:
The item in question that will drop a bomb pickup every 2 rooms when you beat Satan with Cain in the game is the BOMB BAG. Unlocking items in the game usually requires completing specific achievements or goals, such as defeating certain bosses with specific characters. Once unlocked, the bomb bag provides the player with a steady supply of bombs, which are essential for both combat and discovering hidden rooms within the game's procedurally generated levels.