It's Kamikazetime for Blue Dragon (anime season 1)! Review #2
So much so that it is considered a plot twist that killer bat never talks, as shu himself pointed out late,r after finding out who killer bat is "that's why he never talked".
Realms of Valeron - Chapter 16
You see, thirty years ago the dread warlock argonash the undying tried to breed his own army of giant killer bats, in order to use them to launch a war on the floating city of naussica.