Unlocking the Lost in Afterbirth

This Guide is intended to deliver a fast method (~11 mins) of setting up the Lost unlock in Afterbirth, using a really good seed.

The Seed

W2K9 EGKS - make sure difficulty is set to Normal

I'm not sure where this seed originally came from; I found it from a post by steamuser "thequixowhat", so it's likely he stumbled upon it himself.
Shown here: https://steamcommunity.com/app/250900/discussions/0/490124466468258582/

This guide will simply give a quick layout of the rooms with this seed so you can finish the first 3 parts of the run in under 11 minutes easily.
The last part of this guide has a cheat sheet, so if you just want to get right into it and don't need every little tip then just skip to that.

What you have to do to unlock the lost in order (est. time taken with this method)
1. Die with Isaac to a Mulliboom (0:21)
2. Die with Magdeline to one of your own bombs in the Catacombs/Cavers (3:20)
3. Die to Mom's foot with Judas (7:30)
-
4. Die to Satan with Azazel (not included in this guide, as you MUST NOT use a seed for this part of the process). It's recommended to let the 2nd part of the Satan boss fight kill you though, as all his attacks count as Satan, whereas bombs are present in the 3rd phase (and the 1st phase is The Fallen)

Die to a Mulliboom as Isaac

1. From the starting room, go right one. Kill everything in this room, feel free to take damage while you kill everything, just don't die. If you have more than half a heart left, stand in the fire until you have half a heart.
2. Go back to the starting room, and then go up until you reach the room with the Mulliboom (pictured). Stand still. The Mulliboom will reach you first, and you will die to it.

Die to your own bomb as Magdalene

Using the same seed - W2K9 EGKS - we will suicide to our own bomb as Magdalene.
We need to be in the catacombs/caverns to do this, though.

1. From the spawning position, move straight up. We don't need the item from this floor, it won't save time, so just clear the room with the Mulliboom and then kill the Lord of the Flies. Take the growth hormones and move to the next floor.
2. Go down, clear this room. Take the bomb that spawns from clearing
3. Go left, and clear this room. Use a bomb to blow up the rocks and the chest in the top right corner. A skeleton key will drop, which we can use to go back to the previous room and open up the Item Room. There will also be 2 bombs that drop when you clear this room; keep these.
4. Get polyphemus from the item room.
5. Go to the next room, hard left. The fire in the top left of the next room has a soul heart.
6. Go left again, you can one shot the toxic flies in here. Then go straight down two more times, clearing the rooms or using bombs to open doors (but you need to keep at least one!)
You won't need any of these pickups that drop either.
7. The boss room, which contains The Haunt, to your left. If the little haunts drop sould hearts it's ok to take them. When you kill The Haunt, take the Box. It will always drop the Temperance card, which you'll need on the next floor. If you have any Soul hearts left over, get rid of all of them on the spikes. Then go to the next floor once you only have red hearts remaining.

8. Thanks to temperance, this part is really easy. Now that you're in the Catacombs, don't leave the starting room. Use the card and sacrifice hearts until you have half a heart left. If the blood donation machine blows up, just grab the IV bag and use that until you have one heart left. Then, drop one of your bombs and let it kill you. Easy!

Die to Mom's Foot as Judas

1st Floor
1. Input the seed again and start a Normal run as Judas.
2. Go up, in the room with the Mullibooms you can just spam tears without moving and it will kill the mulliboom and kill everything.
3. Enter the boss room and use Book of Belial. Kill Lord of the Flies and take the item.

2nd Floor
1. Go down, clear this room. Go left, clear that room and bomb the chest in the top left. Use the skeleton key to get Polyphemus from the item room that you already passed.
Go leftx2, get a soul heart from the top left fire in the room where you'll one shot the Ragling. Go left again and then down until you get to the boss room.
2. Enter the boss room and use Book of Belial. Kill The Haunt and take the box, grab whatever you want. You won't need Temperance. Feel free to take any soul hearts; we'll have an easy time getting rid of hearts before Mom.

3rd Floor
1. To your right, there is an item room where you can get a little haunt familiar. I believe this is useful but not necessary. It will only save time if your aim is not great in rooms.
2. Go straight down from the starting room and you'll eventually get to the boss room. In the room with the poops, you can just shoot the Squirt without moving and it'll die before it charges you.
3. Enter the boss room and use Book of Belial. Kill Polycephalus, sometimes you can just shoot downwards without moving and it will stay there and die. Take the Jesus Juice.
4. If you get the Devil Room, take it because there are black hearts in there. But be careful; there is also a little devil enemy that shoots you in there. I don't know the name of this enemy.
5. Go to the next floor

4th Floor
1. From the starting point, go two rooms right, then down two rooms.
2. The Item Room has a Teleport, if you get lucky this might be better than Book of Belial but I don't use it.
3. Go right from here and follow this one-way route until the end where you'll go up a little bit and get to the Boss Room. There is a shop on the way, you can buy a soul heart there if you want.
4. Enter the boss room and use Book of Belial. Kill The Stain, take the items and go to the next floor. If the angel room spawns, it'll have The Mitre (red heart pickups may be soul hearts)

5th Floor
1. From the starting room go straight down. In the room with the Floating Knights (the floating heads with brains), Blow up the rock and open the chest. It has Crickets Head which is a good damage item.
2. Keep going down until you get the option to go right. The item room has the unicorn horn active but I prefer Book of Belial.
3. Go right twice then down twice, and you'll get to the boss room.
4. Enter the boss room and use Book of Belial. Kill The Cage and take the magic 8 ball, and take the Hermit card which it drops; we'll need this to do the next floor faster.

6th Floor - Time to suicide to Mom's Foot!
1. Use The Hermit card to go to the shop, skipping two rooms.
2. You can buy the soul heart from the shop here, but if you dont have any bombs buy the bomb. Then go right until you get to the L shaped room with the Greed Heads. Instead of clearing this room, blow up the door that leads downwards with a bomb and leave it.
3. Go down twice, when you get to the room with the screaming enemies blow up the door on your right and skip it if you have a bomb. The enemies get one shot though so if you dont have a bomb for this room its ok.
4. Once you get to the room with the spikes, run over them until you have one full heart left or half a heart. Careful, these spikes do a whole hearts damage, but that's ok; so does Mom's Foot.
5. Enter Mom's Boss Room. Remember that you'll spawn at the bottom of the room, so walk up straight to the center so she instantly steps on you and you die. Well done!

Die to Satan as Azazel

Don't use a seed for this.
You have to complete it on your own new run, so the only walkthrough tips I can give you are
1. Play Well
and
2. Get Satan to the 2nd phase and let that kill you, because The Fallen doesn't count as Satan and the 3rd phase has bombs that don't count as Satan either.

If you die before Satan or input a seed for Azazel, you'll have to do everything from the start.

The Only Part of This Guide that You Need / Cheatsheet / Video

Here is a video of me doing the first 3 parts of The Lost unlock with this seed and explaining what I'm doing.

Below is a cheat sheet for this walkthrough that leaves out all the details and just gives you the directions.

Isaac
Take damage on the right, up twice and die to Mulliboom

Magdalene
Floor one: go straight up to get to the boss room.
Floor two: go down then left, get skeleton key from bomb chest to open Item room. Go all the way left then all the way down to get to the boss room. Take the temperance card that drops when you pick up the Box.
Floor three: use the temperance card to get rid of all your hearts except half of one. Use a bomb and blow yourself up.

Judas
Floor one: go straight up to get to the boss room
Floor two: go down then left, get skeleton key from bomb chest to open Item room. Go all the way left then all the way down to get to the boss room.
Floor three: go all the way down to get to the boss room.
Floor four: go right x2, downx2, and then keep going right until you can go up. go up to get to the boss room.
Floor five: go straight down, until you can go right, then go rightx2 and then downx2. Get The Hermit card when you pick up the magic 8 ball from the boss room.
Floor six: use the hermit. go right, until you get the L shaped room. Blow up the door going downwards, then go down x2 and then right. Take away all your hearts except one on the spikes in the room just before the boss room.
Enter the boss room and run to the center to die to Mom's Foot.

[Ti_Hayden, Melee Enthusiast]

Instruction unclear: I own repentance

[LeonardoAB]

USING SEEDS MAKES YOU DON'T BE ABLE TO UNLOCK ANYTHING

[Gamer]

i died more then once do i have to do it over?

[burn1none]

this guide is obsolete. anyone owning Afterbirth this method does not work

[Super Awesome Banana]

You actually don't need to do all of this anymore. In Afterbirth you can now simply die in a Sacrifice Room while holding the Missing Poster trinket and it will work.

[BreadTM]

can i have a new afterburth seed plz

[hotsadboi]

I did all what u say, but lost didn't unlocks.

[Aihana Etsu]

Good job. Now change your guide to the new method to unlock "Lost". Simple use the missing poster and die in sacrifice room.

And Everyone reading this. Don't waste your time doing this and Google the correct way.

[Sensei Le Roof]

Not that it matters as much if the seed doesn't work, but I think you mean a golden key instead of skeleton key. Skeleton key is an item that gives you 99 keys.

[Bombastic]

it's this fix? i make the same with other seeds (Isaac,Magdalene and Judas) and then with Azazel no seeds, i did all right and didn't work :/