While playing the Binding of Isaac, I should have died when fighting Monstro II, but instead I got a message on a blue screen saying (I think): "Today is a bad day to have a castle". It gave me half a heart back and changed Isaac into a demon for about 5 seconds. Is that normal, and if it is what item is it?
As Yves's comment points out, the only item that is even somewhat similar to what you describe is the Whore of Babylon. The item itself looks like this:
You can get it either by picking it up during your playthrough or by choosing to play as Eve. If you have this item and you have exactly half a heart left, it gives the message "What a horrible night to have a curse..." and turns you into a demon figure like in the following picture:
As the wiki article states, you revert to your original appearance when you leave a room with more than half of a heart.