 Once a room is cleared of monsters , it will remain clear , allowing the player to re @-@ trace their way through the level , though once they move onto the next level , they cannot return . Along the way , the player can collect money to buy power @-@ ups from shopkeepers , keys to unlock special treasure rooms , and new weapons and power @-@ ups to strengthen their chances against the enemies . The player 's health is tracked by a number of hearts ; if the character loses all their hearts , the game ends in permadeath and the player must start over from a freshly @-@ generated dungeon . Each floor of the dungeon includes a boss which the player must defeat before continuing to the next level . On the sixth of eight floors , the player fights Isaac 's mother ; after defeating her , Isaac crawls into her womb . Later levels are significantly harder , culminating in a fight against the heart of Isaac 's mother on the eighth floor . An optional ninth floor , Sheol contains the boss Satan . Winning the game with certain characters or by certain conditions unlocks new power @-@ ups that might appear in the dungeon or the ability to use one of the other characters . The game tracks the various power @-@ ups that the player has found over time which can be reviewed from the game 's menus . 
