Getting both key pieces in the same run will unlock Dad's Key.

The Key Pieces are gotten by going to an Angel Room and beating the Angel mini-boss.

Mega Satan - Beat The Chest or The Dark Room once to unlock Key Piece 1 and Key Piece 2.

To access The Dark Room, you need to beat Satan and touch the chest while The Negative is equipped.

This item will always drop after the Mom fight once unlocked.

Dark Room: Killing Satan in Sheol 6 times will cause The Negative passive item to be unlocked.

To access The Chest, you need to beat Isaac and touch the chest while The Polaroid is equipped.

The Chest: Killing Isaac in The Cathedral 6 times will cause The Polaroid passive item to be unlocked.

Sheol can also be reached by using the 'We Need To Go Deeper' item in The Womb II/Utero II/Scarred Womb II.

Note that these levels can be reached earlier than this if a devil or angel room appear when you kill Mom's Heart (they will only contain the stairway or beam of light).

The beam of light leads to the Cathedral.

Beating It Lives will make a beam of light and stairway appear.Cathedral/ Sheol: Kill Mom's Heart 10 times, at which point Mom's Heart will be permanently replaced by It Lives.Boss Rush: Kill Mom in 20 minutes or less and enter the large crack that appears in the wall.

Binding of isaac chest