2 bats before being aggravated and one after the fact
Bats are level 60 monster found in Ssuorrvlos' Lair (aka Halloween Dungeon). Bats can be aggravated from an incredible distance, sometimes while they are not even visable to the player. They can also use a drain attack to suck HP from the player at long range though they will fly in closer for short range attacks regardless.
Bats have 75,000 HP and 9,000 MP which they use for attacks like Drain. At close range bats can do 2,800 damage and they have a defense of 2,200. Bats can also drop the following items at the following drop rates.
Stygian Essence is dropped at a 0.007% drop rate.
Mark of the Ninth Year is dropped at a 0.009% drop rate.
Hallow Armor specifically is dropped at a 0.01% drop rate.
other level 55 armor is dropped at a 0.03% drop rate.