the-binding-of-isaac-rebirth
The Rebirth Wiki states that you merely need to complete your hardmode post-it with The Lost.
However, recently I heard that the unlock actually requires the hardmode post-it for every character to be completed.
Which of those two things is actually true?
It turns enemies on screen into poop. Does not work on bosses.
Before:
After:
I know it makes your familiars bigger, which I assume also means they do more damage. If you're curious for items in general, I would recommend looking the items up on this item database, as they have most of the items up there and will (hopefully) continue adding them.
Best Answer
You "only" need to beat Mom's Heart, Isaac, Satan, ??? (Blue Baby), The Lamb, and Boss Rush on Hard Mode as The Lost.
As explained on the Achievements page (#156), too. You can trust the official Wikia.
EDIT:
A guy on Reddit has obtained Godhead achievement and he states that you need to complete Hard Mode with The Lost.
I can't find any proof about completing Hard Mode with every other character.