Unstable, dangerous or noxious chemicals has been suggested, but let's go into a few examples:
Chlorine Triflouride Hypergolic with just about everything - including glass, sand, concrete and water. Maybe the alchemist has it as a cleaning agent to really sterilise their apparatus - what are they working with that needs it? There's a famous except from Ignition! that describes a ClF3 fire:
"It is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes."
ThioacetoneExtremely smelly, and very unpleasantly so.
Recently we found ourselves with an odour problem beyond our worst expectations. During early experiments, a stopper jumped from a bottle of residues, and, although replaced at once, resulted in an immediate complaint of nausea and sickness from colleagues working in a building two hundred yards [180 m] away. Two of our chemists who had done no more than investigate the cracking of minute amounts of trithioacetone found themselves the object of hostile stares in a restaurant and suffered the humiliation of having a waitress spray the area around them with a deodorant. The odours defied the expected effects of dilution since workers in the laboratory did not find the odours intolerable ... and genuinely denied responsibility since they were working in closed systems. To convince them otherwise, they were dispersed with other observers around the laboratory, at distances up to a quarter of a mile [0.40 km], and one drop of either acetone gem-dithiol or the mother liquors from crude trithioacetone crystallisations were placed on a watch glass in a fume cupboard. The odour was detected downwind in seconds.
Flouroantimonic acidA "Superacid", so reactive that it's difficult to even find material to store it in.
HF-SbF5 is an extremely corrosive, and toxic substance that is sensitive to moisture. As with most strong acids, fluoroantimonic acid can react violently with water due to the exothermic hydration. Only hydrogen fluoride can be used as a solvent for the acid, given that an aqueous solution can not be used. Heating Fluoroantimonic acid is dangerous as well as it decomposes into toxic fluorine gas. The only method of containment involves storage in a PTFE container as glass will dissolve upon contact. Safety gear must be worn at all times when handling or going anywhere near this corrosive substance. Fluoroantimonic acid can eat exposed flesh down to the bone while reacting violently with water present in human blood cells.
DimethylmercuryDimethylmercury is a mercury compound that allows mercury to be absorbed into the bloodstream through the skin. It is toxic in very small quantities, causing acute mercury poisoning that is often fatal, although it can take several months to kill. There is a famous incident of a chemist Karen Wetterhahn who died from getting a drop on her gloves, which penetrated through to her skin.
The toxicity of dimethylmercury was highlighted with the death of Karen Wetterhahn, a professor of chemistry at Dartmouth College, in 1997. Professor Wetterhahn specialized in heavy metal poisoning. After she spilled a few drops of this compound on her latex glove, the barrier was compromised, and within minutes it was absorbed into her skin. It circulated through her body and accumulated in her brain, resulting in her death ten months later. This accident is a common toxicology case-study and directly resulted in improved safety procedures for chemical-protection clothing and fume hood use, which are now called for when any exposure to such severely toxic and/or highly penetrative substances is possible (e.g., in chemical munitions stockpiles and decontamination facilities).
HydrazineHydrazine is a widely used rocket propellant, both in monopropellant and hypergolic rocket systems. It's also a strong reducing agent. Really, Hydrazine's got it all - it's toxic, carcinogenic, corrosive, highly flammable, explosive in air at a wide variety of concentrations, and hypergolic with a wide variety of common materials including cellulose. The safety protocols around Hydrazine mean that just fuelling up a satellite with it costa about $100,000. When people talk about 'green' rocket fuels, what they really mean is 'anything but fecking hydrazine.'