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?
3 Answers
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.
Requirements
The Godhead is unlocked by defeating The Lamb, ??? and The Boss Rush on Hard Mode with the Lost.
I can't find any proof about completing Hard Mode with every other character.
9No. You just need to complete the Lost's post-it on Hard to unlock Godhead.
And since I can't comment, I'll clear up a misconception in this thread in my post instead: You do NOT need to complete every single thing on Hard to get Real Platinum God. Just the Lost's post-it and the Mom's Heart fights must be done on Hard, and the rest can be done on Normal. (Though many people will still do everything on Hard for completionist sake)
1I have just verified the previous answers on my save.
To obtain "Godhead" you will only have to complete your post-it on hardmode as the Lost - it is not necessary to do so as any other character. (I have only completed Mom's Heart as everyone else in hardmode.)
