Question: the binding of isaac afterbirth plus how to unlock the lost?
Answer: The lost can be unlocked with seeded runs at any step of the process except for the final one. Isaac must die from a Mulliboom in the basement/cellar. A very easy seed to do this is CD2H NNRN, run around hurting yourself with the spiders, there's gonna be 3 mulliboms in there. Keep in mind you have to be in Hard.