Final answer:
The poison used by the man in black on Vizzini in 'The Princess Bride' is iocane powder, a fictional substance presented as odorless, tasteless, and highly deadly.
Step-by-step explanation:
The poison used by the man in black on Vizzini in The Princess Bride is called iocane powder. In the story, this is a fictional substance which is said to be odorless, tasteless, and highly deadly. The main answer to the question lies in the scene of a battle of wits between the man in black and Vizzini, where the man in black challenges Vizzini to determine which cup of wine has been poisoned. This explanation is part of the plot where the man in black, who is actually Westley in disguise, tricks Vizzini by poisoning both cups, as he has built up an immunity to the poison.