The Binding of Isaac: Repentance

In order to unlock this, you need to reach a new area added in Repentence called Home. In order to reach home you must first kill Mother a new end boss found by taking the new alt path and completing certain things on the second of each floor. Once you have defeated Mother for the first time you will unlock a new door on Depths 2. After unlocking this once you make it to Depths 2 from now on there will be a marked skull somewhere on the floor. Blow up this skull to receive a Fool card after the Mom fight use the fool card in order to take one of the Picture items and put it in the door. Doesn't matter which you take because it will remove it from your inventory. Once entering this floor and making it to the boss room there won't be a boss but instead a new item called "Dad's Note." After picking it up you will begin ascending all the floors you came down. You will start taking 1 full heart of damage on these floors just like the Womb. At the very end of basement 1 you will arrive at Home, proceed to the last room and open the chest at the end of Mom's Bed to unlock the red key. Don't forget to use the red key in order to unlock the secret door outside of Moms room. From now on Once you do this the chest will contain a random mom item instead of Red Key. In order to unlock the tainted characters following this you must leave a trinket in a boss room or item room before entering the floor after mom. Once you are in the ascent return to the room you left the trinket in on the exact floor and it will have transformed into a Key Shard allowing you to open the red key door. IMPORTANT NOTE: DO NOT TAKE "LITTLE BAGGY" if you plan to unlock a tainted character in your run as you will lock yourself out of being able to get a key shard since it counts as a card item.