I know the typical rule that if you don't lose any hearts (or red hearts if you have red AND white/silver/black hearts) you can get a Devil Deal Room after a boss fight, but something odd happened the last time I played.
I was playing as Eve (with the curse activated) and I only had the standard 3 red hearts. So I was battling one of those snake thing bosses on the first floor (with both the curse and Monstro's Lung which I had gotten earlier) and I lost a whole heart at one point but I still got a Devil Deal Room.
I don't know if I got it because I defeated it pretty fast or because the Eve character just has that attribute, but I'd like to know why so I can strategize when I play again.
Thanks!
Best Answer
According to the Rebirth wikia, there is only a 1% base chance for the Devil Room door to open. This chance can be increased in various ways:
This means that if you get damaged (this regards ONLY red hearts) during the boss fight you only decrease the chance to open a Devil Room, but there's still a small chance to find it anyway.