A few weaknesses are specified in the rules, such as that of lycanthropes and a few undead creatures to silver weapons. AD&D, I believe, adds some creatures that can be harmed by cold-forged iron weapons. To prevent the party from easily preparing for a couple of obvious eventualities, we need more than that, though. There are lots of possibilities for weapons that might be required to damage particular creatures.
- Stone, either generally, or a particular class (sedimentary, igneous, metamorphic) or a particular kind (limestone, quartz, marble, flint, obsidian...)
- Precious or semi-precious gemstones, from jasper to diamond.
- Wood, either generally, or a particular type (pine, oak, holly, yew, willow, cedar, etc.) Perhaps the wood must come from a particular part of the tree (heartwood, bark, limb less than a year old) or grown in a particular location, such as a graveyard or the site of a fire.
- Bone, either generally, or a particular bone (femur, jawbone) or from a particular creature (human, elf, dragon...)
- A particular metal (copper, tin, lead, gold) or alloy (pewter, bronze, brass, electrum.)
Not every vulnerability needs to be of the "what can we hit it with to kill it?" form. Weaknesses that have other effects are often more interesting and force players to think farther outside the box.
Aversion or revulsion: The creature cannot or will not touch some material or object, or take some action. The classic examples of lycanthropes' fear of wolfsbane, and the vampire's inability to cross running water or enter a home without being invited are examples of this sort of weakness. Depending on its exact nature, an aversion or revulsion may be used to exclude a creature from an area, confine it, or repel it. Aversion or revulsion can be physiological (wolfsbane's effect on lycanthropes or running water on vampires) or psychological/symbolic (the vampire's prohibition against entering uninvited.)
Compulsion: The creature is drawn to or obsessed with something to the point of being unable to resist. For example, folklore has it that vampires and/or witches may be kept at bay by throwing a handful of grain, or by covering windows with screens, and the creature is compelled to stop and count the kernels of grain or the holes in the screen. (This was once used to hilarious effect in an episode of The X-Files.)
Debilitating: The object of weakness drains the creature's abilities or incapacitates it in some way. This could include all manner of effects, such as calming or putting to sleep, disabling or weakening an offensive power, blinding, deafening, slowing, disorienting, mesmerizing, physically weakening, preventing the use of escape powers such as teleportation or ethereal movement, canceling defensive powers such as invisibility, and negating immunities.
Possible triggers for these weaknesses are many. In practical application, factors may be combined, such as using herbs and music, or inscribing a circle of salt accompanied by a chant, or a ritual performed under a certain aspect of the stars. These components might be effective only in combination, they might be effective separately but reinforce each other, or one part might be effective while the other is superfluous.
Symbols and talismans: Typically an item with a connection to the creature type or a particular individual. It may be a specific item (the sword that ended the normal life of a now-undead creature, a medallion owned by its mate, etc.) or a class of items (holy symbol, olive branch, mirror.) It may trigger any of the effects above.
Plants: Herbs, especially strongly aromatic ones, are often credited with potency against evil spirits and monsters. Their power is often released by burning, in the form of incense, smudge pots, and the like. Burning sage is said to cleanse an area of evil. Flowers, leaves, or other parts of particular plants may act as talismans against particular creatures.
Animal or human parts: Horns, hide, hair, blood, organs, and body parts of animals, people, or monsters might have power over some supernatural creatures.
Minerals: Salts, metals, ashes, soils, sands, oils, powders, and crystals may be efficacious against some monsters. Lines of salt are commonly used as barriers to prevent the passing of supernatural creatures.
Sounds and music: Particular tones or tunes can affect monsters in many different ways. A Gregorian chant might well keep blasphemous undead at bay. In the original Legend of Zelda game, the sound of the flute stripped one boss of its immunity and rendered it vulnerable to attack. The Golem in the first Dragon Warrior game was put to sleep by the music of the fairy flute.
Rituals: Various rituals may have power to bind, repel, banish, or weaken monsters. Drawing runes or pentagrams in the dirt is a ritual, as is reading from holy scriptures, sacrificing a black ram, or blessing all the homes in a village. Rituals often require one or more other factors, such as materials or a particular celestial aspect.
Celestial bodies: Night and day, phases of the moon, tides, or alignments of stars and planets may affect supernatural monsters. Powers may wax and wane or even vanish altogether at certain critical conjunctions. Sometimes celestial alignments are simply propitious times for the performing of rituals or for the harvesting of herbs or other materials for greatest efficacy.
Vulnerabilities should be assigned carefully and deliberately, not randomly. Where myth and folklore are silent, reason and common sense should guide. Some examples of my own, below:
- A dwarven revenant has a compulsion to count coins and can only be harmed by weapons of stone.
- A vampire elf will not touch wood, and can be slain only with weapons made of the heartwood of a mallorn tree.
- An undead creature whose curse is penance for murdering a faithful lover recoils at the sight of a red rose; weapons can harm it if tipped with rose thorns.
- A glass golem is weakened by the sound of a particular very high note, becoming riddled with cracks and susceptible to ordinary weapons.
- Wraiths and spectres are rendered semi-physical by the smoke of a burning mixture of sage and other herbs, and susceptible to full damage from silver weapons while in this state. They seldom remain in the area, though, unless confined by a line of salt.
- A creature that skulks in the holds of ships and preys on sailors cannot abide fresh water, and cannot set foot on soil or stone. Being splashed with alcohol negates its immunity to normal weapons, so it purposely throws kegs and bottles overboard; disappearance of rum and wine aboard a ship is often the first sign of the monster's presence.
- Gargoyles can be harmed only by cold-forged iron. Pouring salt on their wings renders them unable to fly for one full turn.
- Elementals become vulnerable to normal weapons if they lose contact with their element.
- The wights of a certain burial ground will not attack a person visibly brandishing a holly bough, the token of the order of knights who opposed their wicked clan in life.
- Shadows cannot cross running water, just like a vampire, and are vulnerable to weapons of black obsidian.