The initial possession should cause some damage and be painful, sort of like ripping out your essence. From the outside it could look like somebody was being shot by arrows from many directions, one at a time causing their body to convulse to each new hit. It is basically a fight of mental strength, the new spirite fighting the current owner. They should have a chance to resist, albeit a low one. Once done they should be weakened (lower HP) whether or not the spirit took over.
And after a few hours, you could even say that your *true* spirit that got thrown out had to learn the ways of the spirit world so it could try and get back to their body. So the time of possession could depend on the intelligence of the person. A very intelligent person would only be gone for minutes perhaps. They being the natural owners of the body have an easier time re-possessing it so they get even more bonuses to re-entering their own body. Should be a chance to failing again though. Go to Comment
I would posit that the root and leaves have a similar effect when accidentally consumed, but are differently used in the rituals or secret recipes. The 'Actual Effects' are what this plant really does and can go wrong (with side effects of course).
I like the idea of special preperations giving the ability to draw a certain spirit. Say, add a piece of the intended spirit or have or use one of thier possesions. That would work great into a story line of a warrior tribe with ancestor worship. They would have a ritual of cutting the hair or pulling toenails when one of them dies and then mixing it in when taking the beast weed, to try to call on the skills and might of their ancestors. They would probally have a good sized patch of beast weed they were tending to. Go to Comment
I did not remember ever seeing this one before or after the move. I think the toxin idea is interesting. I think the side effect of your sould got booted and something else comes is a bit off, but useful.