The rare version is also called a "a shrouded bat" and drops: Frozen Gnome Eye.
Upon it's death, instead of two vampires, you get one named vampire: Malvus Darque.
Malvus Darque treats the death spot of the named shrouded bat as it's spawn point, so if you drop/lose aggro/die, it just sits where it popped (instead of where the bat's spawn point is).