An evil presence is expressed through increased monster activity.
Seen It A Million Times, Needs A Better Title, Needs A Better Description. In JRP Gs and related game genres and works of fiction, the rise of a Big Bad is usually accompanied by a sudden appearance or increased presence by monsters throughout the land. Why? So the hero can fight them, of course! This can be explained in-game in a number of ways, such as the monsters being Sealed Evil In A Can or the existing population simply having a greater presence due to increased aggression or intelligence. Sometimes no reason is given. But you can bet that in every JRPG you'll find that one NPC who says something like "things were great until the monsters came." Just to clarify the point of this: The focus isn't that the monsters appear, but that they coincide with or are a result of the presence of the Big Bad or a generally "evil" presence.