This is a follow-up question for Finding Nymphs Before Predators and covers the terrifying possibility of Nymph-infused superhumans. So....
Recap: Magic bleeding through from an alternate reality combines with built-up residual life force (which is found in cemeteries, warzones, battlefields, anywhere where lots of things die) to form Nymphs. Nymphs appear to be human women in their prime and are unaffected by hot and cold (making them able to stroll through the Arctic and wade in lava flows) not to mention able to breathe air and water with equal ease, speak and understand any human language, and even know any animal or plant in their environment. Despite all of this knowledge, Nymphs still have childlike awe and love of nature.
Nymphs are born clad in leaf clothing (a la Tinker Bell and her fairy friends), being naive, peaceful, curious, and with a deep longing and belief: the longing is to find their purpose, and their belief is that fate is real (ie. that they will find their purpose in the course of their journey).
All Nymphs go on a journey after being "born," both to see the wonders of nature for themselves and to find their purpose. However, this is where my question comes in. You see, Nymphs have a natural ability to "return to nature" by being eaten, which infuses their consumer with their potent natural energies and enhances them.
As an example, a Nymph-infused tiger is supernaturally stealthy, becoming invisible in shadows, and is fast and strong enough to A) catch up to a running horse and B) rend that armored knight on the horse's back. It also has all the nature-related knowledge and human intelligence of a Nymph.
That being said, the problem with someone eating a Nymph is clear. A natural runner would be able to outrun a horse at top speed, while a well-built warrior would have the strength of a gorilla. Their senses would be enhanced (actually, as put above, their everything would enhance, but only one thing will be the most enhanced) and they'd have both the ability to speak and understand any human language and the knowledge needed to survive out in the woods.
Never mind a human's main advantage over other animals: endurance. Biologically speaking, animals have the advantage over us in every area except this one. Well, Nymph-infused humans will have that advantage over everyone else. That runner mentioned above? Yeah, there's no way your cavalry can catch up to him, even with a head start. You'd have to catch him napping. And you can pretty much forget about wearing down that well-built warrior; no sane army would take him on.
So, my question is: How Can an Early Medieval Society Deal With The Danger of Nymph-infused Humans?
Specifically, how can kings prevent an upstart (noble or peasant) from finding a Nymph and becoming a serious threat? (Please note: the previous question concerns finding Nymphs before predators do, so Nymphs don't become monsters and terrorize the populace. However, the system used to do this also makes it really easy for someone to find a Nymph if they want to....)
One Final Note: Thanks to everyone for all of your thoughtful comments! Each and everyone was useful, but in the end, I had to pick one.