The Pair of Bat Wings is a Grimoire material and starts to drop upon starting The Ancient Art. They drop (1/20) from Cave Bats in the underground.
ΒΆ Summons
Owl: 2 Owl Feathers, 1 Pair of Bat Wings
Bat: 1 Owl Feather, 2 Pairs of Bat Wings
Poacher: 1 Owl Feather, 1 Pair of Bat Wings, 2 Scorpion Tails