It is a bit weird how, in D&D, Vampires have a bunch of weaknesses: garlic, holy symbols, running water, etc. Few other monsters, have any such weaknesses; a few are blinded by strong light, and that’s about it.
The reason that vampires have such weaknesses, of course, is because they are a part of real-world folklore. We have a lot of stories about vampires. We don’t have as many stories about, say, displacer beasts.
Of course, inside a game world, this distinction disappears… and so does the reason for vampires to be unique in their pile of weaknesses. Many monsters could have such weaknesses. This idea seems to have a lot of traction. There is even an entire PC class built around the idea that monster have weaknesses that can be exploited.
The problem, of course, is that players read monster manuals. If the weaknesses are listed there, players will be aware of them whether or not their PCs are.
One possible solution to this is the Secret Weakness.
The material enclosed in the box below is released via the Open Game License.
When PCs either research or first encounter a new monster, determine whether the creature type has a weakness by rolling (or choosing from the options on) the following tables.
Table 1: Weakness Type
1-3 No Weakness
9 Material weakness
10-11 Energy weakness
12-13 Magic weakness
20 Roll twice
1-10 Minor fear (DC 12)
11-17 Major fear (DC 16)
18-20 Catastrophic fear (DC 22)
When presented with the object of their fear, a creature must make a Will save at the listed DC. If the creature succeeds, it is shaken for a single round. If it fails, it is shaken for 2d4 rounds. If it fails by 5 or more, it is frightened for 2d4 rounds instead.
Some common objects of fear include: Darkness, Light, Fire, Heights, Water, Magic, Faith, or a specific type of creature, object, or social situation. It is best to choose one which fits the monster appropriately.
1-2 Cold Iron
9 Other (common, such as steel or stone)
10 Other (rare, such as ice or blood-soaked wood)
If a monster has a material weakness, that material ignores the monsters damage resistance, if any. If the monster has no damage resistance, it take double damage from weapons made of the material to which it has a weakness.
(roll or choose)
If you roll an energy type for which the monster has Energy Resistance, reroll. Monsters with a weakness to a type of energy take double damage from that energy type.
(roll or choose)
1-10 Minor weakness (-2 Spell Resistance, -2 on Saving Throws)
11-17 Major weakness (-5 Spell Resistance, -4 on Saving Throws)
18-20 Catastrophic weakness (No Spell Resistance, -6 on Saving Throws)
Choose a subschool (or school) of magic to which the magic weakness applies. Against that school, of magic the monster takes the listed penalties. Alternatively, spells of the appropriate school could have an additional effect such as double duration or 150% damage when used against the creature.
(roll or choose)
1-10 Minor taboo (DC 12)
11-17 Major taboo (DC 16)
18-20 Catastrophic taboo (DC 22)
A taboo is a cultural prohibition against an action. Taboos can be widely varied, ranging from touching corpses to cannibalism to looking at the sky to surrendering in combat to speaking to creatures of other races to using tools. Choose an appropriate taboo for the creature type. A creature forced to break a taboo must make a Will save at the DC listed above or be Dazzled, Shaken, or Sickened (choose one appropriate to the taboo broken).
(roll or choose)
1-10 Minor ban (Dazzled, Shaken, or Sickened, as appropriate)
11-17 Major ban (Cowering, Dazed, Fascinated, or Nauseated, as appropriate)
18-20 Catastrophic ban (Incapable of willingly breaking)
A ban is like a taboo, but is enforced magically or physiologically rather than culturally. There is no save against a ban. The effect of a ban lasts as long as the condition holds true. Examples: Orcs have a minor ban against being exposed to bright lights while Vampires have a catastrophic ban against crossing running water.
Discovering a weakness
PCs may discover weaknesses of monsters through either trial and error or research. The knowledge DCs should be set in such a manner to reflect both the rarity of the monster and the frequency with which the monster’s weakness comes into play. For instance, knowing that orcs don’t function well in sunlight may only be a DC 5 check in a campaign where there are many surface-dwelling (but nocturnal) orcs, but this would be a more difficult check in a campaign where orcs are rare and live only in the Underdark.
Weaknesses and CR
Weaknesses – by their very nature – make a monster weaker. In many cases, the weakness is relatively trivial. However, in other cases, a weakness makes overcoming a monster significantly easier. Unless the weakness changes the very nature of a monster (for instance, giving worgs a taboo against biting an intelligent creature), the CR on a monster should not be adjusted the first time that the weakness is brought to bear on that monster by the PCs. Instead, they should reap the full benefits (including experience points) of exploiting that weakness. If the weakness is a particularly debilitating one or one which is easy for the PCs to exploit, it may be advisable to lower that monster’s CR by one once the PCs are aware of its weakness.
Edit: Additional weaknesses in comments. Add your own!