Vecna

In the Stranger Things 4 finale, it's revealed that the Upside Down - which has been established as a Dark World version of Hawkins inhabited by monsters as an infectious Hive Mind - was originally a barren wasteland with (presumably) non-malicious indigenous lifeforms. That is until Vecna, later revealed to be Henry/One, his body mutated when Eleven banished him there after he killed the other subjects at Hawkins Lab, used his god-tier psychic powers to create the Mind Flayer and transform the world into the hellscape it is now.

Example of:
Sealed Evil in Another World