A weapon is needed for a climactic battle against an otherwise invincible wolf monster. In principle it could be any random thing, like the traditional silver, but we want something based in science. It should be a real chemical with actual biological properties, and something that takes effect fast enough to be used as a weapon.
Characteristics of the Chemical
An answer must be a real chemical satisfying the following requirements:
- Secrecy The chemical should not be present in everyday life in the real world, including both modern times and ancient times, or else it would be discovered too easily and the monster would never get a chance to be invincible. It cannot be in food or in the air in anywhere near the effective dose.
- Effectiveness The chemical needs to have some unusual biological effect when applied in the correct dose. This effect will serve to destroy the monster while nothing else available could, so it has to be something that cannot be achieved in any easier way. It should probably cause a loss of homeostasis in some biological system and that homeostasis can turn out to be critical to the monster's invincibility. The effect does not need to be harmful to real animals.
- Nontriviality The effect of the chemical should not be peripheral; removing the monster's hair or changing its color would never seem like a plausible way to destroy the monster's invincibility. Samson's weakness to having his hair cut is not a good example of the kind of weakness we're looking for.
- Availability The heroes need access to the chemical, so it either needs to be something that can be manufactured without sophisticated equipment, or it needs to be a chemical that is so stable that it could have survived for 300 years without climate control only to be discovered when it is needed. In this world there is nothing more advanced for doing chemistry than a typical kitchen, but in the past all sorts of advanced chemistry would have been possible.
- Weaponizability The chemical needs to be effective as a weapon, which means a small dose is enough. The ability to be absorbed through skin would be good, or it might be inhaled as fumes. If it could be effective by being coated onto an arrow, that would be excellent. It also needs to take effect quickly, ideally in less than a minute.
Characteristics of the Werewolf
The monster is a mammal that resembles a wolf, but it is the size of a tall and heavily built human and has a tendency to walk upright. Therefore answers that take advantage of either wolf biology or human biology are acceptable.
In spite of having real mammalian biology, the monster is a terror that overwhelms all other attempts to defeat it. There is some part of it that allows it to fight through any injury and recover from anything, but the exact mechanism is irrelevant. If we can find a suitable chemical, we can say that whatever organ or system is affected by the chemical is somehow critical to the monster's invulnerability. The chemical itself doesn't need to be deadly, since the monster can be killed by conventional means once its invulnerability is removed.
The monster's invulnerability has no effect on ways the chemical might be delivered. The monster's skin can be broken, and the monster cannot stop itself from breathing if the chemical must be inhaled. The monster has no special resistance to chemicals being absorbed through the skin.