Culpeper gives henbane to Saturn, and it is the least strained attribution in his book. Saturn in the planetary scheme is cold, heavy, deadening and associated with death; henbane is a narcotic poison. The signature and the fact agree for once, and Culpeper says so without his usual argument.
The plant's more famous association is with the flying ointment of the witch trials, a fatty preparation of tropane-bearing plants, most often given as henbane, belladonna and mandrake, said to have been rubbed on the body or on a staff and to have produced the sensation of flight. The pharmacology here is sound in principle: tropane alkaloids are absorbed through skin, and scopolamine delirium characteristically involves sensations of flying and falling and encounters with figures that seem entirely real. Several twentieth-century researchers tested such preparations on themselves and reported exactly that.
What is much weaker is the historical claim that this was a widespread practice rather than a widespread accusation. The ointment recipes come overwhelmingly from demonological literature written by prosecutors and from confessions extracted under torture, which is precisely the class of evidence that manufactures whatever it is looking for. Ronald Hutton and others have argued that the ointment is better understood as an element of the learned stereotype of the witch than as a reconstructable folk technique. The chemistry would work. Whether anyone was actually doing it, at scale, is not established.