I love the concept! I love the permanent and distinct change to the realm. This will be super fun to bring into any game.
My only question is the intent of the Malign Swamp. It doesn't appear to be directly part of the myth so my guess would be for just use as a site in the game? I don't think it's something that needs to be explicitly said, but it is something I'm second guessing as a Referee as to the best way to use it.