A deal with the Devil can be gained through multiple means with different actions affecting the probability of one appearing.
Base chance is always 1%
- Have a Pentagram +20%
- Have Book of Revelations +35%
- Have Book of Belial +25%
- Kill a beggar +35%
- Take no RED heart damage against boss +35%
- Take no RED heart damage for the whole floor +99%
After the above factors the chance of a Devil room appearing is multiplied by the following.
- Devil room spawned on previous floor x0.25
- Devil room spawned two floors ago x0.50
Once you have access to a Devil room there will be one or two items with either 1 or 2 hearts underneath. You must sacrifice the number of heart containers underneath the item you wish to take, do this twice in one run and you will pop the achievement.