Binding of Isaac rebirth unlocking the chest

If you want the complete list:

• The Womb: Kill Mom Once
• Boss Rush: Kill Mom in 20 minutes or less and enter the large crack that appears in the wall.
• Cathedral/Sheol: Kill Mom's Heart 10 times, at which point Mom's Heart will be permanently replaced by It Lives.
  • Beating It Lives will make a beam of light and stairway appear.
    • The beam of light leads to the Cathedral.
    • The stairway leads to Sheol.
  • 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). Sheol can also be reached by using the "We Need To Go Deeper" item in The Womb II/Utero II/Scarred Womb II.
• The Chest: Killing Isaac in The Cathedral 6 times will cause The Polaroid passive item to be unlocked. This item will always drop after the Mom fight once unlocked.
  • To access The Chest, you need to beat Isaac and touch the chest while The Polaroid is equipped.
• Dark Room: Killing Satan in Sheol 6 times will cause The Negative passive item to be unlocked. This item will always drop after the Mom fight once unlocked.
  • To access The Dark Room, you need to beat Satan and touch the chest while The Negative is equipped.
• Mega Satan - Beat The Chest or The Dark Room once to unlock Key Piece 1 and Key Piece 2.
  • The Key Pieces are gotten by going to an Angel Room and beating the Angel mini-boss.
  • Getting both key pieces in the same run will unlock Dad's Key.
  • Collecting both key pieces or using Dad's Key will unlock the giant door in The Chest or The Dark Room, allowing you to fight Mega Satan.
• Blue Womb (Afterbirth DLC): Kill It Lives in 30 minutes or less.
• The Void (Afterbirth+ DLC): Kill Hush in The Blue Womb. Once you've reached The Void for the first time, there is a chance of a Portal to The Void appearing after these bosses:
  • 5% chance after Mom (and only if you have a lot of damage or items)
  • 10% chance after It Lives
  • 15% chance after Isaac (Cathedral) or Satan (Sheol)
  • 20% chance after ??? (Chest) or The Lamb (Dark Room)
  • 50% chance after Mega Satan (Chest/Dark Room)
  • 100% chance after Hush (Blue Womb)
• Downpour (Repentance DLC): Defeat Hush 3 times
  • To access Downpour, open the locked door at the end of The Basement or its alternates.
• Dross (Repentance DLC): Defeat all bosses in Downpour, appears as an alternate chapter to Downpour.
• Mines (Repentance DLC): Defeat Hush 3 times
  • To access the Mines, bomb the wall in the last room of The Caves I/II or its alternates.
• Ashpit (Repentance DLC): Defeat all bosses in the Mines, appears as an alternate chapter to the Mines.
• Mausoleum (Repentance DLC): Defeat Hush 3 times
  • To access the Mausoleum, open the door that appear in The Depths I, its alternates, or the Mines II that takes 2 hearts to enter (except when playing The Lost).
• Gehenna (Repentance DLC): Defeat all bosses in the Mausoleum, appears as an alternate chapter to the Mausoleum.
• Corpse (Repentance DLC): Defeat Hush 3 times
  • To access Corpse, open the door at the end of the Mausoleum II/Gehenna II; it is opened using Knife Pieces 1 and 2 that you collect from Downpour and the Mines respectively
• Ascent/Home (Repentance DLC): Defeat Mother in Corpse once
  • To access the Ascent (which leads to Home) you need to open the door in the starting room of The Depths II/Necropolis II/Dank Depths II; you must have The Polaroid, The Negative, or the Faded Polaroid to open the door. This leads to Dad's Note which starts the Ascent.
    • For the Polaroid or the Negative, this means you have to defeat Mom then pick up the item and teleport out of the boss room.