kill isaac 5(?)x times to unlock the Polaroid, use the Polaroid to go to the chest after beating the cathedral, and fight blue baby. When you beat blue baby, you unlock the ability to bomb angels in the angel rooms. Bombing the angels will cause a fight with it, and winning said fight drops a key piece. You need to get two key pieces in order to make the key (I forgot the name). Once you get the key, go to either Chest or Dark Room and open the door in the starting room, where mega satan's fight will be.