Bats is deadly for an unadvised player who steps on Snow Biome in LostWorld Dimension at night without torches or lights. They only spawn if the place is very dark and player is not holding a torch.

You can't kill these bats, you just need to take care before changing the torch at your hand with another item at night.

Don't forget: NEVER cross snow biomes at night! or at least, hold a torch in your hands all the way!