Note that you normally cannot leave the boss room after defeating Mom you will need to have some way to teleport out before you go in and fight Mom. This will consume the item and unlock the strange door. Defeating Mother for the first time will cause a strange door to appear at the beginning of Depths II in all subsequent playthroughs of the regular path. At the end of the Antibirth path, you will fight Mother.
Navigating the Antibirth levels is tricky you may want to look at a separate guide to learn how to acquire the Knife Pieces and reach the Corpse.
This leads to an alternate set of levels, based on the Antibirth fan mod. Here, you will fight a powerful boss named Hush. On subsequent playthroughs, defeating Mom’s Heart (or its replacement, It Lives!) before the timer reaches 30:00 will cause a path to the Blue Womb to open up. Unlocking tainted characters is a long process, reserved for the very end of The Binding of Isaac’s unlock order.
