An alchemist takes reagents harvested from magical creatures and plants, and blends them to create their stock.
Alchemy as a field of study only took off around 540 AD, but is said to have been around for thousands of years in folk remedies and traditional medicine.
However, it was only after the Starfall that it became a necessity to research. With the Starfall, natural ambient magic decreased to the point that the previous casting type healing fell to near uselessness. However, the monsters raiding their settlements were not only their downfall but also their salvation. Some of the new creatures had parts that could be used to create potions with near the same level efficiency.
Still, while it seems like the next step would be to research this, it took centuries for alchemy to take off. Why? Even today, alchemic studies are dangerous, especially to the inexperienced. Weak potions can be created with no risk, but the life saving healing that civilization needed at this point? The risks were high even without taking in regard the raids. No one had the knowledge base to get far enough, and those who made it far enough were likely to die before completing their research. Furthermore, after a couple centuries of this, most people wrote it off as a risk to the town. Most alchemists were driven off, or forced to operate in secrecy for almost no gain.
It was only after Yuʀu Lalaj created the standard healing potion in 526 AD that things began to turn around. Yuʀu gathered a following and quite the fortune over the next two decades, and his students became the driving force behind the rejuvenation of the profession.
Today, alchemists and their wares are the backbone of civilization. Their healing potions and explosives support adventurers in the field, and create medicine to counter plagues. Most are either hired by the Academia or the Guild.