Getting towards the end of the traveling day, the scout sees a perfect resting spot well protected from the elements. To get to it the group is forced to struggle through some vines with thorns not unlike those on a rose. It is inevitable that most or all the group will receive scratches but it is a small price to pay for the comfort and protection of the perfect campsite. Funny that the scratches have trouble scabbing over.
That night the effects of the scratches start to take their toll. The thorns hold a toxin that slowly works into the bloodstream causing a deep meditative sleep to take hold. The more scratches the more powerful it is but everybody with scratches would be affected to some extent. Even those on watch can’t fight the effects and soon they will be drowsing.
While the sleep that takes hold of them more then doubles their healing rate, it also causes them to be unaware of the creeping vines that start crawling towards them. The vines will wrap themselves around their victims and extract their blood for food.
The vines are intelligent and if somebody on watch is not asleep then the vines will creep along the ground and go after those affected. The vines can innately tell who is affected and their mental state (aware or not). The toxins will keep the victims healthy for a very long time so they can continue to provide food.
1)Smart players, if they escape, will realize how useful it could be if they can extract some of this toxin for future use. Whether to heal themselves or to knock somebody out. Does one try and transplant one to sell?
2)Perhaps the toxins have a fountain of youth effect as well. Wonder who else may be laying around out there. Do they wake them or rob them?