Overview:

The event only concerns the overworld; Nether and End stay the same and aren't changed. (configurable)

Normal mobs don't spawn, although they won't burn during the event.
Sound system, mobs rely on sound to track the player. The more sound the player makes, the more their radius extend (for the duration of the noise).
Shader during the event that makes it so dark areas are pitch black.
Mushrooms turn to darkness-repealing lightblebs during the event. They will turn back to mushrooms afterwards.
The event stops naturally at dawn.