Poisonous plant and classic Witch garden flowers. All of these plant while lovely are indeed toxic. For more information read my book "Garden Witches Herbal". There you will find a chapter devoted to Gothic Witch Garden Plants. -Ellen Dugan
Helleborus, which can cause tinnitus, vertigo, stupor, thirst, a feeling of suffocation, swelling of the tongue and throat, emesis (vomiting), and catharsis, bradycardia (slowing of the heart rate), and finally collapse and death from cardiac arrest. Pretty though.
Monkshood: Aconitum nagarum, dark purple form. I can't pin down a variety or a source for seeds, sadly. You will have to do a bit of sleuthing if you want one. Excellent for a poison garden; not so good if you have kids around.