How do the new “skull” challenge rooms work

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

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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?

Best Answer

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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