In "Prophecy", when going onto a space that has a villain on it, are you required to fight it?
The short answer is yes, if you end your movement on a space with a revealed monster on it you must fight it.

This is set out in the rules, under Adventure Cards, the two parts that govern this are the following:


  You are always affected by the Adventure cards located in the space where you end your movement.


and


  If you encounter a Creature you must battle it (unlike using an Opportunity,
  fighting a Creature is mandatory).