Miranda thorns are an assassination tool invented by Ma'ar. Very rare, they contain a highly toxic contact poison that causes hallucinations and extreme pain before death. There is no antidote. Healers and mages can only buy the victim a short amount of time and a clear head while they await death.
Conn Levas used Miranda thorns on Urtho. Having only seen three cases of Miranda thorn poisoning during the entire length of the Mage Wars, the healers were at a loss. Urtho hung on as long as he could to allow the evacuation of the area around his Tower. His death triggered fail-safe spells that destroyed his Tower and all the surrounding countryside, one half of the Cataclysm.