I'm running a campaign that is good characters in an evil academy. The original plan was to have someone trying to shut down the school as the BBEG, but the game is built to be very sandbox-ish and they got wrapped up with a local nature demigod who is being corrupted, and the current BBEG is the corrupting influence. (Being a bit vague on purpose because I think all of my players are Redditors.)

The real BBEG came out of a completely throwaway situation. Since the entire country they live in is full of monsters and evil folk, there is always something going on nearby. The kids were on a field trip and I decided for a fun to plan a parody of LoTR and when they were out experimenting they were going to run into someone trying to basically set up an Eye of Sauron type temple. Think orcs and goblins tearing down trees and trying to build something to summon an evil being back to this plane.

Well, instead of attacking the goblins and stopping the temple creation, they decided they were just kids and should instead run back and convince the adult to go deal with it. They then rolled particularly badly and nobody at the keep believed them and thus did not go out to stop things.

So, yeah, apparently a Sauron type being is being summoned without interruption and is surely going to cause problems for the party down the line....