Aconitine - Kills the pain, then kills your heart

• analgesic (killer of pain) and cardiotoxin (causes arrhythmias) found in plants belonging to the genus Aconitum (otherwise known as aconite, monkshood, soldier's cap, helmet flower, or wolfsbane)
• wolfsbane sounds awesome (see also: henbane)
  
• the hood/cap/helmet name is the result of the characteristic helmet-shaped flowers that species of Aconitum produce
  
• its cardiotoxicity prevents it from being used therapeutically as an analgesic
• extracts of these plants are used in traditional Chinese medicine to treat pain and inflammation, a great example of how natural/herbal remedies are often more dangerous than the drugs they replace
• is occasionally intentionally used to poison people
• is thought to bind to and open voltage-dependent sodium channels in the heart and central nervous system, resulting in the disruption of electrical conduction in the heart (producing arrhythmias) and pain signals to the brain (producing analgesia)
• used to create animal models of cardiac arrhythmia, which help researchers to develop new antiarrhythmic agents (yay!)
  

Gutser UT et al. Mode of antinociceptive and toxic action of alkaloids of Aconitum spec.. Naunyn Schmiedebergs Arch Pharmacol. 1998 Jan;357(1):39-48.