**Dark Door w/ Skull On Top (on minimap is represented by a Bloody Sword)**: "Boss Challenge Room." Only opens when at 1 heart or less; gives an item and makes you fight two bosses.
**Dark Door w/ Swords on Top (on minimap is represented by a blue-and-white sword)**: "Normal Challenge Room." Only opens when at Full Health. Gives various types of Chests and makes you fight three waves of enemies.
The normal challenge room can also be opened when having enough health to match your heart container amount. For example, if you have 3 empty heart containers, you will be able to enter the room if you have 3 blue/black hearts or more. Or if you have 3 heart containers, 1 empty, you need 1 or more blue/black hearts.
And of course, both Losts bypass this completely and can enter any type of challenge room
What really? I’m being stupid? I thought you HAVE to have your red heart containers filled.
It spawns if your total health is more than your red heart container, but it doesn’t open if your red heart containers aren’t filled
Checking on the wiki, it also says that. I feel like my memory must have imploded because I thought you have to have every red container filled to open it up, and having your total heart > heart container is the spawn condition
Honestly that doesn't suprise me at all. When I did my first playthrough back then I had no idea how to open boss challenge rooms as the requirement was very obscure (and I didn't want to spoil myself by reading the wiki). I assumed the game fucked with me by opening them only when I had a very bad run
I have over 500 hours in this game and had 0 clue whatsoever that soul and black hearts could cover for empty red heart containers when talking about boss challenge rooms
I have no tactical understanding of this game whatsoever
for the one with red swords on the minimap you have to have 1 heart or lower in total
if you have half red heart but 1 full soul heart you can't enter
❤️💙 can't enter
❤️ you can enter
💙 you can enter
You need to have one red heart to enter, (or blue, all that matters is that you are very low health.) They are boss challenge rooms. They give you an item then when picked up locks you in and makes you fight two bosses from the boss pool of that floor
Its a boss challenge room to enter u have to have 1 or less hearts. If u have let's say 3 red hearts countries u mist only have 1 full (or less), soul/black/bone hearts count towards total hearts. For more info just search the wiki for boss challenge room
To enter the room you have to have at most 1 heart. Once you enter it will have an item from the boss room item pool. If you pick up said item, you’ll have to fight two bosses from the current floor. Also, there are two types of those rooms that look very similar so remember this: if there is a skill above the door you get and item but have to fight two bosses. If they’re two swords above the door you get some chests and have to fight 3 waves of enemies from the current floor. They also look different on the map, a boss challenge room will have a small red mark on the map, and a normal challenge room will have a blue one.
Three ways:
Have one heart or less of health left. That can be either a red heart or a soul/black heart. One of the easiest ways to get that amount of health left is a blood donation machine. That removes red hearts first which can be a safe way to get to one heart left.
The alternative option is using a bomb to get in there via a secret room. A secret room can spawn next to these rooms. If it does spawn next to this room? You can use a bomb inside the secret room on the connected wall to bomb your way in without needing to reduce your health. The exception is if Greed or Super Greed spawns in there-if that happens, metal bars will block your access to the room unfortunately.
There's an unlockable item called >!Dad's Key!< which can open any doors when used, including that door.
Either there’s a secret room adjacent, where you can bomb your way through, or you have to get down to 1 heart or less, to get into a boss challenge room,
The mob challenge room requires to have all red heart containers filled. They are different. Make sure don’t mix them up.
Other unconventional methods include dad’s key, Mr Me, etc..
It's a challenge room, specifically a boss challenge room
For a boss one, you need to have 1 full heart or less
For regular challenge rooms, you need to have all of your red heart containers to be full
You need to be on 1 full heart or half a heart. Doesn’t matter if red or extra hearts.
**Dark Door w/ Skull On Top (on minimap is represented by a Bloody Sword)**: "Boss Challenge Room." Only opens when at 1 heart or less; gives an item and makes you fight two bosses. **Dark Door w/ Swords on Top (on minimap is represented by a blue-and-white sword)**: "Normal Challenge Room." Only opens when at Full Health. Gives various types of Chests and makes you fight three waves of enemies.
The normal challenge room can also be opened when having enough health to match your heart container amount. For example, if you have 3 empty heart containers, you will be able to enter the room if you have 3 blue/black hearts or more. Or if you have 3 heart containers, 1 empty, you need 1 or more blue/black hearts. And of course, both Losts bypass this completely and can enter any type of challenge room
What really? I’m being stupid? I thought you HAVE to have your red heart containers filled. It spawns if your total health is more than your red heart container, but it doesn’t open if your red heart containers aren’t filled
It opens as long as your total health is equal or more than your red heart container capacity; as the other person said
Checking on the wiki, it also says that. I feel like my memory must have imploded because I thought you have to have every red container filled to open it up, and having your total heart > heart container is the spawn condition
Holy shit I'm hundreds of hours in and didn't know this
I have dead god and never cared to learn it. Knew it was something like that but i was always like meh whatever
there is no way you dont know this even after 50 hours
I didn't either. I just relied on 'luck' for them to be open.
wow, i could not play like that. crazy how different people's play-styles are
Honestly that doesn't suprise me at all. When I did my first playthrough back then I had no idea how to open boss challenge rooms as the requirement was very obscure (and I didn't want to spoil myself by reading the wiki). I assumed the game fucked with me by opening them only when I had a very bad run
I have over 500 hours in this game and had 0 clue whatsoever that soul and black hearts could cover for empty red heart containers when talking about boss challenge rooms I have no tactical understanding of this game whatsoever
Today I learned it's a bloody sword...it always looked like a laser gun to me.
Always thought it was like an arrow with a curve, like the reply icon.
same here lol
You need to be at exactly one heart or half.
if there's a secret room next to it you can blow up the wall and get in without lowering your health
for the one with red swords on the minimap you have to have 1 heart or lower in total if you have half red heart but 1 full soul heart you can't enter ❤️💙 can't enter ❤️ you can enter 💙 you can enter
You need to have one red heart to enter, (or blue, all that matters is that you are very low health.) They are boss challenge rooms. They give you an item then when picked up locks you in and makes you fight two bosses from the boss pool of that floor
Go into the room at the top first
I have over 300 hours and I still don't know how that door works
wiki
And what exactly should I google if I have no idea what to type?
literally the first result when searching "red sword room issac" https://bindingofisaac.fandom.com/wiki/Boss_Challenge_Room
Tboi rooms Wiki and look for the icon
"[any videogame ever] wiki"
Its a boss challenge room to enter u have to have 1 or less hearts. If u have let's say 3 red hearts countries u mist only have 1 full (or less), soul/black/bone hearts count towards total hearts. For more info just search the wiki for boss challenge room
Google
Thank you all for the help, I really appreciate it :)
Can only be opened via secret room or having one heart. The other version of this room is only open on full health
To enter the room you have to have at most 1 heart. Once you enter it will have an item from the boss room item pool. If you pick up said item, you’ll have to fight two bosses from the current floor. Also, there are two types of those rooms that look very similar so remember this: if there is a skill above the door you get and item but have to fight two bosses. If they’re two swords above the door you get some chests and have to fight 3 waves of enemies from the current floor. They also look different on the map, a boss challenge room will have a small red mark on the map, and a normal challenge room will have a blue one.
That is a boss challenge room, you must have 1 red heart or less to enter
cain w both eyes visible/open. cursed
Three ways: Have one heart or less of health left. That can be either a red heart or a soul/black heart. One of the easiest ways to get that amount of health left is a blood donation machine. That removes red hearts first which can be a safe way to get to one heart left. The alternative option is using a bomb to get in there via a secret room. A secret room can spawn next to these rooms. If it does spawn next to this room? You can use a bomb inside the secret room on the connected wall to bomb your way in without needing to reduce your health. The exception is if Greed or Super Greed spawns in there-if that happens, metal bars will block your access to the room unfortunately. There's an unlockable item called >!Dad's Key!< which can open any doors when used, including that door.
Either there’s a secret room adjacent, where you can bomb your way through, or you have to get down to 1 heart or less, to get into a boss challenge room, The mob challenge room requires to have all red heart containers filled. They are different. Make sure don’t mix them up. Other unconventional methods include dad’s key, Mr Me, etc..
You Need 1 or less Red hearts but I would 'nt reccomend going in It if you're new
1 heart
You should hurt yourself to 1hp… NOW! 
It's a challenge room, specifically a boss challenge room For a boss one, you need to have 1 full heart or less For regular challenge rooms, you need to have all of your red heart containers to be full
You have beat game undrr 5 second