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 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.

As Yves's comment points out, the only item that is even somewhat similar to what you describe is the Whore of Babylon. 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 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: