Fomentation of the Form

There is a commonly known truth that individuals who place themselves in danger regularly, battling monsters and coming into mortal peril, enjoy certain benefits as a consequence of their lifestyle. Throughout history, there are many stories of groups who quickly rise to power due to their harrowing experiences. And yet, these stories always seem almost impossible to believe without seeing them because they often describe these powerful figures, commonly referred to as adventurers, shrugging off what should be crippling wounds in a matter of hours. A short reprieve to patch their wounds can restore these figures, allowing them to draw on an inner well of health. A nights rest will often see them completely healed, with only scars to show for the mortal injuries of yesterday, if that.
The Fomentation of the Form is a ritual designed by medical researchers to create these effects of great healing in individuals who are still green to a hard life. In moments of desperation, the ritual has been used on lords and ladies, and other important individuals, to allow them to move through warzones with some heightened measure of life expectancy.
It is believed the the bodies of adventurers go through two others changes, a Fomentation of the Mind, and a Fomentation of the Spirit, which enable them to intuit things at a heightened pace from mostly combat experience, and to become resistant to most methods of non-magical control or persuasion, respectively. These other two transformations have been unable to be replicated by magical means, perhaps due to the difficulty of testing if these effects have taken hold.

Discovery

The stark difference between legendary heroes and the masses from which they originate have long been swept under the rug as a mere result of advanced training and natural talent. However, when researched with scrutiny, it became clear to interested parties that, whatever the cause, there is a clear physiological difference between an adventurer and a normal man. At first, it was assumed that this difference is perhaps an inherited trait. After all, many adventurers (the ones that survive) that go on to have families often produce more of their ilk. Unfortunately, this can be disproven by consulting records. Individuals who go on to become heroes have frequently arisen from circumstances in which it is unlikely that they would have remained for so much of their lives if they possessed some great innate power from birth. The sheer number of adventurers who have come from abusive families, slavery, or even noble lineage all demonstrate a dramatic shift in the pace of their learning and their abilities to heal, beginning only at seemingly random points in their lives. It was at the summit of these questions being asked that a powerful mage who became taken with the topic decided to take matters into their own hands. So, the mage began to hire mercenaries. He would send them on missions of increasing difficulty, waiting for signs of them healing up quickly between missions. Then, he would orchestrate a disaster, capturing the fledgling adventurers and experimenting on them until their spirits, minds, or bodies gave out. If they lost their spark of adventure, they would be released, memories modified, with years of their lives missing. If they died, then they needed to be replaced.
The researcher completed his work, but was eventually found out. Despite his brutality, it seems that his rare mercy on those who survived his trials was his downfall. One of them had their memory restored. Nearly 20 powerful adventurers descended on his laboratory, destroying his work, and him with it. Some of them, clearly mages, Obliterated his name from all record, which is why he has been referred to anonymously in this record. And yet, the mage researcher had apprentices, and not all of them had been present at the time of his demise. They scattered and went underground, seeking patrons to protect them in exchange for their knowledge. Some were found, but unfortunately the awesome force that had assembled to destroy the mage was made of fickle adventurers. While some sought revenge for longer, most of them scattered to the wind, tempted by other mysteries, monsters, and treasures.

Utility

The Fomentation of the Form doubles the physical fortitude of an untrained combatant, and allows them to heal injuries completely with a night's rest.

Access & Availability

The Fomentation of the Form is dangerous. There are many warriors who have inherited the bitterness of the adventurers who were lost to fuel the research. Many of them hate the idea of the strength of "true warriors" being bestowed upon undeserving folk. 
Despite the stigma, most royals will maintain at least one mage who possesses knowledge of the Fomentation of the Form for emergencies. It is a good way to make oneself feel better about the survival of their families or children in the case of assassination. These mages are often kept secret, and comfortable. They are not frequent members of court. Some nobles even keep their "Boosters", as they are called, in other towns entirely.