A perennial common throughout Europe and most of the United States, variously called simply Aconite, often Wolfsbane or Monkshood (Eisenhut in German). The plant favours shaded or partially shaded areas, grows roughly 3' to 4' high at full maturity, and bears along its long, thin stalk dark-green leaves, forking into 5 parts with rough edges and coarse veins. From mid-July to early September, shoots bud into the dark purple flowers from which the plant gets its name, cupping over the stamen at the top like a monk's cowl, generally clustering towards the top of the stalk; certainly among the more beautiful and striking plants in a respectable herb-garden.
The plant's derivatives have a long history, and indeed even today it produces one of the most potent plant toxins known; 1 mg of the extract can be enough to stop the heart, rivalling both digitalis and belladonna. The extract, made from the roots and stem, was often used by the Anglo-Saxons to tip their arrows both in war and for hunting wolves (thus another of its names, Wolfsbane). When ingested or otherwise introduced into the bloodstream in humans, common symptoms include, at first, anxiety, a pricking sensation under the skin, dizziness and blurring vision, then nausea and vomitting, dilated pupils, and slowed breathing and pulse, finally partial muscle paralysis and death. It is no surprise that Vergil in his Georgics (Georg. II.152) mentions its elimination as one of the traits of his idyllic Italy.
There are few treatments for aconite poisoning; effects are rather quick. If it is swallowed, as usual, drink plenty of milk and induce vomitting, though it often does as much damage coming back up as it did going down. Atropine or lidocaine can be administered, if available.
There are no culinary, and few medicinal, uses for Aconite. The ancient world certainly saw little use for the plant except in ensuring quick dynastic successions. Medieval monasteries were a bit more daring, using the oil as a topical ointment or linament to relieve symptoms of rheumatism and arthritis (still practiced in a limited form today), and a much-diluted version as a strong emetic; the leaves were boiled in teas to treat migraine and headaches. Modern research hasn't been able to do much more with it, though the derivative Aconitine is used in much the same way as digitalis to treat heart failure.
It is not surprising that a long tradition of folklore has attached itself to Aconite. An ancient Greek story claims that it first grew before the gates to Hades, and gained its poison from the dripping saliva of the three-headed hound Cerberus. Already in Rome it was viewed as the plant of Hecate and her magic, continuing an association with witchcraft into the mediaeval period. The early Saxon use against wolves lead to the eventual belief that aconite was an effective ward against both werewolves and, later, witches.
As a final note, all of the above refers to one particular genus, the one most commonly found in Europe and with which I have the most experience; others exist, and often (such as in certain Chinese variants) have nowhere near the same effect. The leaves can be relatively harmless; in a fit of adventurous madness, or perhaps migraine-induced desperation, I did boil the leaves into a tea and drink the vile, bitter-tasting result, escaping with a somewhat-improved head (after half an hour of vomitting) with no other particular effects. On the other hand, I still remember clearly and with vague feelings of guilt stumbling into my garden one morning to water my herbs, only to encounter one half-eaten Aconite plant and one very dead, very unfortunate rabbit. (I'm sorry...I really am very sorry...). Caveat Hortulanus