It is a tiny plant, the Beast Weed. Looking at it, it resembles a clover-leaf, yet its leaves are deeply red on the under-side. Under the ground, it is a single thick root with many fine little roots.
The Beast Weed scents and tastes sweetly. But if some animal consumes it, it often gets extremely aggressive, to the point of truly fighting to death (few animals do that). Worse, its power and reflexes are supernaturaly enhanced, even small animals can get dangerous. (Remember Monthy Python’s killer bunnie? Jumping after one’s throat and biting yer head off? THAT kind of dangerous.) Luckily, the Beast Weed is very rare.
Actual effects: this little plant produces a strong toxin, slightly magical, that sends you to the Spirit World, and draws any spirit that might possess your body into it. Usually a spirit of the same kind. Because the spirit came to our world unwilling, and even little cute animals can have steroid-pumped representatives in the spirit world, out comes a killing machine! (Not to mention if the spirit is drawn against its will.)
Even sentient beings usually draw similar spirits to them. This can also mean spirits of deceased fighters, ready to fight anyone, sometimes for any or no reason at all. Your quiet little friend may become a berserker, wielding his sword with incredible skill. But all kinds of spirits can possess you, it is truly random and cannot be relied upon.
Knowledge and possible uses: Druids know of this plant. While it is against their nature to destroy anything completely, they carefully remove the plant from most places it could be found by chance, by animal or man. Druids but do not use it.
Those of shamanistic bent know it well, and they know the risk. At dire times they may seek it, leaves and root. Carefull preparation, special rituals and above all knowledge of spirits lessens the risk, and may call a given spirit into your body.
For those that know about it, and stand alone against a mightier enemy, the Beast Weed may be a last wildcard, a chance when all else fails.
Note: after a few hours, the spirit returns to its world, and your spirit can return into your body. If it is still living, of course.
(And one more note: Truly powerful spirits may leave something of their essence behind…)