The Villain II

The person who replaces Villain either because Villain Sucks, I'm Better Than My Boss, or Villain Can't Villain. This happens because the writer thinks the villain of the story needs to be replaced due to canonical/plot reasons, has too many plot-related problems to continue being the main antagonist (i.e they have Power Flaws, have a Vines Backstory, their ideals have been abandoned, invalidated, or become impossible to fulfill, or have real-life problems) and must be removed from the story altogether or must become a Retired Character, or just becomes plain uninteresting to The Audience. This is where Villain II comes into play. They give Hero a new goal and/or challenge, continue the plot, and hopefully don't piss everyone off by being introduced.

This means Villain II might have a new lair to hang out in, extending Hero's journey and keeping the story alive. And with a new character comes new abilities. Hero is, at this point, forced to destroy all knowledge they have of their enemy and must start all over.

However, Villain II might not be a permanent change as Villain I might Return To Power and resume what they were doing before Villain II came around.