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There's a new kind of challenge rooms, these one:

enter image description here

I've managed to get in three times, always under the Holy Grail effect (pre 1.33 patch it was possible to cross these closed doors while flying), and they have an improved loot, usually an item, and thougher enemies (I faced a smaller Monstro and 3 small Larry Jr.)

What's the prerequisite to enter in here?

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Is it any different from the normal challenge room (where you have to have full health or higher to enter)? – Ian Pugsley May 29 '12 at 1:43
@IanPugsley already tried with full health or higher, it has to be lower, but I don't know the treshold... – Kappei May 29 '12 at 4:42
up vote 3 down vote accepted

It's been labeled the "Boss Duel Room" by the wiki, and can be entered if you either have one hit point remaining, not counting the soul hearts. It will spawn 2 bosses relative to the floor you're on.

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I don't know if this is a bug or not, but I have approached one of these rooms multiple times without having been hit only to see the doors remain closed... – Aubergine Jun 12 '12 at 14:41

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