Bats are a bit trickier than they might seem at first. They come in two varieties: normal and giant. The varieties have the same sort of habits and habitat, but giant bats are a lot tougher than normal ones. Both giant and normal bats are blinded by silence 15' radius, but not by light, and they are generally unaffected by any spells that depend on sight.
What spells do depend on sight? Here’s my list:
- Light
- Continual Light
- Invisibility
- Mirror Image
- Phantasmal Force
- Hallucinatory Terrain
- Massmorph
I almost included projected image, but the image “cannot be distinguished from the original except by touch,” so I’m going to suppose it’ll fool bats.
The fact that bats aren’t fooled by invisibility and most illusions might be useful to characters who want to figure out if something is illusory. Anyone who wants to use bats to detect illusions, however, also needs a good way to interpret their behavior. That could be speak with animals, other magic, or just a lot of familiarity with bats. Vampires, for example, should be able to interpret bat behavior.
Normal Bats
Normal bats wander the first level (or outside) of a dungeon in groups of 1–100, which is also the size of their lair inside a dungeon. In the wilderness, they might have lairs of 5–500. They don’t have treasure.
Normal bats aren’t dangerous by themselves. They can only attack in groups of 10 or more, and the only effect of their attack is confusion, imposing -2 on “to hit” rolls and saving throws, and preventing spell casting.
By the book, each normal bat defeated is worth 5 XP. But a bunch of bats by themselves aren’t a threat, and PCs who decide to fight them shouldn’t get any XP for it unless they are part of a more interesting, and challenging, encounter.
Normal bats are nuisance monsters, but they can complicate other situations. An encounter with bats might draw the attention of other monsters. Or, wandering bats might arrive during a fight with something else. The bats are probably not very picky about who they confuse: they’re mainly just panicked. But there is no save against the confusion effect, which not only penalizes attacks, but makes spell casting impossible. For that reason, vampires will certainly summon bats to harass enemy spell casters.
A ring of animal control is not very helpful for making bats work for you. The effect requires concentration, and it doesn’t control the minimum 10 bats needed to produce confusion. A potion of animal control, however, has a chance of controlling enough of them.
Normal bat reactions are simple, like the animals they are. They will usually panic or flee from danger if it’s obvious how to do so. Unless they’ve been summoned or controlled, they won’t continually confuse someone. Instead, they will confuse characters incidentally, as they try to get out of danger.
Giant Bats
Giant bats are more dangerous than normal bats. They have two full Hit Dice. Their flying movement is 180' (60'). Their morale is 8—good but not great. They occur in groups of 1–10. Their main weakness is that their bites don’t do much damage, only 1–4 points. Still, an average giant bat will have 9 hp and THAC0 18, which is enough to make them at least worrisome for low-level characters.
A giant bat is really big. Ordinarily, 2 HD animals are about 5' long. The DM has to decide if that’s body length or wingspan. If it’s wingspan, then giant bats are just a little bigger than the Indian Flying Fox. Their body is a foot long to go with their 5' wingspan. They weigh about 4 pounds.
Fortunately for the PCs, giant bats don’t usually eat or attack humans or demihumans. But they will if they are extremely hungry. As DM, your default assumption should be that the bats are not that hungry, unless you have reasons for deciding otherwise.
You’ll find giant bats in groups of 1–10 on the second level of the dungeon. On the first level, there might be 1–8. They might appear in larger groups on levels four and five, likely 2–20 and 3–30, respectively. In the wilderness, their lairs will house 5–50 bats. But they don’t have treasure of their own.
Actual tactics for giant bats aren’t sophisticated. If attacked, they flee. Don’t worry about morale. They’re animals with a survival instinct and nothing to protect.
On the other hand, if giant bats attack, they will attack like predators: going after the smallest, most lightly armored target they can reach. Of course, none of that matters if they are controlled by a ring or potion of animal control, or if they’ve been summoned by a vampire.
Giant bats aren’t strong enough to pick up even a halfling, and they don’t have a swoop attack.
Having no treasure, giant bats are a potentially dangerous nuisance monster or a complication of an encounter with another kind of creature, such as a vampire (who can summon 3 to 18 of them!).
Like robber flies, though, giant bats are also dangerous because they look like another monster, vampire bats.
Vampire Bats
Five percent of giant bat encounters are actually encounters with giant vampire bats. These bats can cause those they bite to fall unconscious for 1–10 rounds, so the bats can drink their blood at a rate of 1–4 hp per round. Those who die must save vs. spells or become undead. Unlike ordinary giant bats, vampire bats are always hungry enough to hunt humans and demihumans, but they will still hunt like predators.
The ability to cause unconsciousness with a bite should be worth an asterisk on the vampire bats' Hit Dice, making them worth 25 rather than the normal 20 XP.
When a vampire bat’s victim becomes undead, what sort does it become? The rules mention that it can be a vampire, but they don’t give much other guidance. You could base it on character level, or you could simply decide that certain bats make certain kinds of undead, based on the needs of your campaign.
Apart from vampires, ghouls and mummies are good choices for the results of a vampire bat bite, as there is no other way in the rules for them to be created, other than fiat. (Which is fine! You don’t need rules for undead origins to have great undead in your game.)
