After just unlocking The Lost myself, I figured I would make a guide for it, seeing as there is next to no info about him.
The lost is, as the trophy description says the final secret of the game. The Lost has zero health, meaning you hit once; its game over.
This is the way I used to unlock the lost (worked first try) and I would advice you to try the same. There are video's for it on youtube, but I've read alot of mixed comments about it not unlocking for alot of people, so here's my method.
You will need to do all steps in a row, without dying, unless needed for that step. If you die inbetween steps, you will have to redo the entire process again. However, there is 1 way in which you can somewhat fool the system. If you are on PS Vita/PS4, after each step just upload your save to the server, and if you die just redownload it, and continue from the step you where on.
For those that prefer it that way, I have added a video below showing the way to do it aswell.
What you will need to do to unlock the lost;
1.
Die by a mulliganboom as IsaacFor this one, you can use a seed. Make sure to put the game to hard difficulty and use the following seed - CD2H NNRN* Its on your first door down then left
2.
Get Maggie and die to your own bomb at cave Same for the mulliganboom; you can use a seed for this one aswell. this is the one I used - AP08 77PS (Can be either caves 1, or caves 2)
3.
Get killed as Judas by moms foot For this one I would not advice to use a seed. Reccomended: Upload your save file before you start with judas, as Judas only has 1 heart you can die very easily. That way you can just redownload your save file, and try again.
Just play though the game normally untill you get to moms foot, and die to the foot chrushing you.
4.
Get killed as Azel by Satan (Not the fallen) Again, upload your savefile before you start the run, in case you die.
Another one to NOT use a seed on. Using a seed for this seems like the most common thing for the Lost not to unlock.
This is not possible, unless you have unlocked sheol prior to trying this. Satan is the second form in the boss fight, and the first one (that splits in to 2 enemies, is the fallen)
You HAVE to die to his shots, not the (brimstone) beams.
If all done correctly, The Lost, and the trophy should unlock upon dying with Azazel to Satan.