A quick look at challenges.xml lets you see what achievements do you need in order to unlock a given challenge. That file is my source.
"April fools" challenge can be unlocked by killin mom for 1st time, cant say about other A+ challenges. - Anon
--220.127.116.11 14:19, 6 January 2017 (UTC)Can we confirm how challenges are unlocked? when i start a new savefile, the Afterbirth + challenges aren't unlocked by default.
When doing Challenge #11, I had to go against Satan, not Isaac, and only a trapdoor to Sheol would show up for me, no beam of light to The Cathedral.
I'm quite sure that I didn't beat Mom's Heart on Hard, but for some reason I unlocked it during my last run. Not sure how though.
The challenge unlocks might be based on challenges down as opposed to specific challenges. I unlocked Anzus after doing my firt challenge which was the tank.
You dont forfeit the challenge by passing up the trophy if you have the ability to continue to another stage, you just have to be able to beat that stage, then another trophy will appear.
18.104.22.168 21:10, 23 December 2014 (UTC)
Challenge 26 seems to be unlocked by unlocking Mom's Knife - which, given you start with it, would make sense.
--22.214.171.124 23:22, 3 November 2015 (UTC)
I'm pretty sure the "April fools" challenge is a play on that bee movie meme. It's Binding of Isaac but every boss is the bloat.
Also I've noticed that pills and cards say one pill/card text and activate another pill/card effect.
The challenge is actually pretty easy as well because picking up coins seems to give you hearts. Those hearts seem to have a higher than average chance of being gold hearts, which drop more coins, which repeats the whole process.
Interestingly enough you still need missing hearts to pick up red heart pickups even though they don't fill health.
- It's more of a joke by the devs at the player's expense. Think about it. Everyone hates Bloat, so why not make every boss Bloat? Items, pickups, even the map laughs at you and your attempt to understand the madness. --JokerPixel (talk) 06:40, 4 January 2017 (UTC)
Just checking if it's just me - Ehwaz rune not working on Challenge 34: Ultra Hard. Makes sense, really. -Anonymous
I complete Pokey Mans challenge by killing Delirium (portal into the void spawned right inside me after killing Mom's foot). I read that Ultra Hard challenge can be dealt in that way too. Maybe now any challenge can be completed by killing Delirium? 126.96.36.199 10:30, 22 January 2017 (UTC)
So possession of the negative is not actually required to unlock ultra hard. Killing the lamb and then unlocking the negative and traveling to the dark room is good enough(ie. you killed satan(having already done a sacrifice room run prior and killed the lamb previously) for the fifth time and used a sac room it will unlock upon arrival to the dark room). Uh yeah I know the amount of bullshittery I did to test that was pretty awful(still haven't unlocked it lives! but I now have ultra hard unlocked XD) - anonymous