tetrodotoxin

There is no antivenom for a blue-ringed octopus bite. The only treatment is to be hooked up to a ventilator for 15 hours while the tetrodotoxin wears off so your diaphragm is no longer paralyzed. – WTF Fun Facts

Source: https://slate.com/technology/2015/06/blue-ringed-octopus-venom-causes-numbness-vomiting-suffocation-death.html