The Mask + Heart is a new enemy added in the expansion Wrath of the Lamb. They are commonly found in Necropolis and are found less commonly in Utero. Sometimes hearts can be found on their own in Utero.
It consists of a mask that is immune to damage with the movement pattern similar to the Knights but much faster, and a slow moving heart that will shoot projectiles in a cross pattern when the player comes close to them. The mask itself however can be killed with the Necronomicon.
Heart
A Heart may spawn as a independent enemy in later levels (such as Utero, this may be a bug). It hovers around the room and moves away from the player. When near a heart, it will shoot blood in a + formation.
Behaviour
While the Mask is immune to all forms of damage, the Hearts are the key to destroying them. Killing the heart will also destroy the Mask it's accompanied with.
In the movement mechanic, the mask is more aggressive than the knight, almost the same as the Champion Knight. Once it has the line of sight of the player it will immediately charge at him.
It can be very frustrating if there are 4 of them in one room. You must constantly move and focus on the heart and avoid the masks. If there are rocks in the room it can provide cover against the charging masks. Piercing shots are very useful against that situation.
Notes
A boss variant called the Mask of Infamy can be encountered in the first floor in Necropolis. It has a similar mechanic to this enemy. However, the heart will shoot in eight directions and the mask will not die with the heart, but continue to fight until it is destroyed by repeatedly shooting its backside.
The hearts are immune to the Unicorn Horn and Chariot card. Unknown if it is a bug or intentional.
There is a bug in which the Necronomicon can kill the masks but not the hearts. This gives the heart no purpose.
Despite that both the Heart and the Mask are floating, they cannot float over rocks and pits