Henbane is Hyoscyamus niger, and it is poisonous. That is not a caution attached to an otherwise useful plant; it is the central fact about it.
The active constituents are the tropane alkaloids, principally hyoscyamine, scopolamine and atropine, which are potent anticholinergics. They block the action of acetylcholine at muscarinic receptors, and the resulting syndrome is characteristic and severe: dilated pupils and loss of near focus, dry mouth and skin, flushing, racing heart, retention of urine, high fever, and a delirium marked by vivid hallucinations that the affected person cannot distinguish from perception. The classical mnemonic in emergency medicine runs blind as a bat, dry as a bone, red as a beet, mad as a hatter, hot as a hare. Deaths occur, and children have been poisoned by eating the seed capsules.
The plant is unmistakable at close range and easy to overlook at a distance: sticky, hairy, greyish, with dull cream flowers netted in purple veins and a dark purple throat, and a strong unpleasant smell when bruised. It grows on disturbed ground, middens and old walls.
Henbane belongs to the Solanaceae, the nightshade family, alongside belladonna, datura and mandrake, and alongside the potato, tomato and pepper. The family is one of the most pharmacologically consequential in the world, and its toxic members are toxic by the same chemistry that makes them medically interesting. Hyoscine, still in clinical use, is a henbane alkaloid.
The delirium is the feature that matters most for the plant's history. Unlike the hallucinations produced by other classes of drug, anticholinergic delirium is not recognised as unreal by the person experiencing it: conversations are held with people who are not present, and the memory afterwards is fragmentary or absent. That is a specific and well-documented clinical picture, and it is directly relevant to what the plant was later said to do.