Question: how to unlock the forgotten in the binding of isaac afterbirth plus?
Answer: DEFEAT THE LAMB. Before The Forgotten can be unlocked, you have to have beaten The Lamb, the boss of the Dark Room, at least once. ... 
CHOOSE YOUR CHARACTER WISELY. ... 
BE QUICK. ... 
BOMB THE STARTING ROOM. ... 
GRAB THE SHOVEL AND RUN. ... 
GET TO THE BOSS RUSH AND COMPLETE IT. ... 
GO TO THE DARK ROOM AND DIG UP SOME DIRT. ... 
SUCCESS.