9 Steps to Unlock The Forgotten in Afterbirth + - KeenGamer
The Binding of Isaac: Rebirth on GOG.com
Floors - The Binding of Isaac: Rebirth Wiki Guide - IGN
The Binding of Isaac: Repentance - How To Unlock New Secret Floor / Corpse - YouTube
Unlockable characters in The Binding of Isaac. | Download Table
The Binding of Isaac: Rebirth on GOG.com
The Binding of Isaac Rebirth: Character Completion Marks - GamePretty
I created a table which shows the items you can unlock by killing different bosses with different characters. You can just print it out and check the items you've already unlocked. :
The Binding of Isaac Repentance Patch Notes | All changes, What's New! - Pro Game Guides