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Typically, only special rooms are marked on the map (item rooms, shops, arcades, bosses, deadly sins, etc.).

What's special about the blood room shown below? I tried hurting myself on the spikes, and nothing happened.

Blood room

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up vote 4 down vote accepted

If you damage yourself some inconsistent number of times on the spikes in the room, a chest appears.

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Pay attention that the spikes do a full heart of damage even pre-womb (never tried on womb or later), as I painfully learned by trying it with Cain. – Kappei May 29 '12 at 7:59
Yes, spikes always do a full heart of damage. – murgatroid99 May 29 '12 at 12:46
The spikes in this room can hurt you even when you're flying. – disinter Jun 10 '12 at 15:43

They are great for the nun power-up, you can get free re-rolls.

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Try using the pony power, or the book that gives you a shield, and walk over it. A chest will appear, and you won't take any damage.

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The sacrifice rooms will drop chests if you take enough damage, but if you use items that give mercy invincibility, like iv bag or whore of Babylon. Also, if you have tarot card The Chariot, A pony, White Pony, Book of Shadows, or Numerous other invincibility Items, you will get a chest for free. If you have the Left Hand trinket ( WoTL ) the chest will be a Red Chest.

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