This is an excerpt from the "Pardoner´s tale which is one of the Canterbury Tales written by Michael Young. Three young men depart to encounter Death and kill it. The adversary mentioned is "Death".
Answer:
The correct answer is Death.
Explanation:
The publican refers to death as a great adversary, one everyone should be afraid of.
He says his opponent "has killed a thousand" and "went his way without a word".
He is an inevitable adversary that everyone should be face carefully, but always being prepared, knowing you can meet him any time, and anywhere.
