Shadow Mask

The Shadow Mask is a rare enemy that can only be found in the Sky Palace. As its name suggests, it is an evil floating mask complete with eyes and a nose. The Shadow Mask's most notable trait is that its face is made up of two halves that are black and white, which will split apart when attacked. These two halves will chase after Pit until they are both destroyed. When the mask is whole, it will simply move in a vertical pattern between the floor and ceiling. Each half will drop a small Heart.

Kid Icarus: Of Myths and Monsters
"Has a face like a mask that splits in two when Pit attacks."

Trivia

 * The Shadow Mask's design may be loosely based on the symbol and concept of yin and yang, due to its color scheme.
 * In the game's manual, the Shadow Mask is shown to have eyes with pupils. However, its in-game sprite lacks pupils, having only blank white eye sockets.