Alchemist's Fire

Alchemist's fire is a Brownish-Green and somewhat sticky liquid that is extremely flammable, and has a variety of military uses.

Utility

Alchemist's fire is almost universally a military weapon, used particularly in sieges or large open battles where dense formations of soldiers can be targeted for maximum effect. One of the most horrific uses is to dump in down into gateways filled with soldiers, as they cannot run forward but into your own spears, and they can't run backward as they are blocked by their own people.
Alchemist's fire was also the staple of siege defense against monsters from The Shadowfell, as entire outer walls could be set ablaze, burning any of the creatures bold enough to attempt to get through, or dumping it into their tightly packed hordes for maximum damage. 
Alchemist's fire has also been used extensively as an effective booby trap, and the fire itself is great at setting of chain reactions with other flammable liquids or materials.

Manufacturing

The main barrier to manufacturing alchemist's fire is the ingredients. One of the main ingredients is a type of liquid produced in very deep and thick bogs and swamplands, often called pitch for it's black opaque coloration. Large amounts of pitch need to be concentrated down and mixed with other ingredients while being heated in a manner that doesn't include direct flame. As this process requires a significant amount of raw pitch, only a few boglands on the continent are suitable for it's extraction en masse. In addition, extraction can be expensive, as it is difficult to do without being exposed to disease from the swamp, or accidentally lighting the pitch on fire, wasting massive amounts of it in the process.

Social Impact

Threatening to use alchemist's fire on an enemy is a serious threat, and it's use is considered cowardly by some cultures. However, none can argue with the horrific way in which it kills it's victims, and so it acts as much as a fear weapon than anything else. Watching a friend or comrade being burned alive by alchemist's fire will quickly sap the resolve of anyone, be they soldier, cultist, or peasant rabble. 
due to it's especially painful nature, alchemists fire will sometimes be brought in for the execution of heretics, and other particularly evil criminals.

Access & Availability

Alchemist's fire is relatively rare, and as such expensive, prohibitively so to those without much money. In addition, the ingredients to produce it are only found in sufficient quantities in a few areas of the continent, and so it's availability can also depend on where you are (you can understand why a merchant wouldn't necessarily want to carry a volatile flammable/explosive substance over long distances).

Complexity

While alchemist's fire is not necessarily complex to produce, it is complex to handle and utilize effectively. the substance needs to be stored in pottery, as any kind of material that is not water-tight will eventually ooze the stuff, and the pottery even needs to be lined with fat to prevent seepage. In addition, any flame, or even an over-exposure to sunlight can ignite it, and cause the batch to be wasted... or worse. Finally, when stored in a confined space, it is so violently flammable that it can become explosive. Reports from some storage buildings exist where a pot of alchemist's fire was dropped in a cellar full of pots of more alchemist's fire, causing a chain reaction which blew up the building, killing 22 people. As such, to use it effectively requires extremely safe production and storage facilities, as well as trained people to use it without hurting themselves or others first.

Discovery

The discovery of Alchemist's Fire is generally attributed the Saint Ecclesius's work during the defence of Caprera against a daemon host. It is said that he used whatever ingredients were local and on-hand to make the stuff. Regardless of how he did it, many records indicate that alchemist's fire was used extensively to burn daemon cultists attempting to scale the walls and breach the gates of the city. Many say that the black streaks on the walls can still be seen today, though the locals will tell you that that is moreso due to water damage than to ancient alchemist's fire.