This is for unlocking Tainted Isaac. You need to reach the floor "Home" with either the Red Key or Cracked Key as Isaac. See here for a guide on how to get there in the first place
If it's your first time reaching Home, you'll find the Red Key in a chest. On any other run, you'll need to find it somewhere, or get the Cracked Key one time use item. It could drop anywhere normally, but there is a guaranteed way to get the Cracked Key.
On your way down to the Depths 2, drop any trinket (hold down ctrl by default) in either a boss room or an item room. When you're making your way back upwards, you will find the Cracked Key in the same room wherever you dropped the trinket. Since you always start each floor in the boss room, it's easiest to just drop it there on the way down.
When you get to Home, sleep in Mom's bed, then leave the room. Standing right outside the door, use the Red Key or Cracked Key, and a room will spawn to your left. Go inside and you'll find the tainted version of your character crying on the floor. Walk into them and you'll unlock them to play as.