Mangetsu
Steampunker
Small question. I created the world with Crimson. And in the process of exploring the world, I found a cloud consisting of Demonite Ore. Is it a bug or a modification mechanic?
Sounds like the evil island.
One of the more recent updates added in an "Evil Island", which is the opposite world evil then what the world started with, so, in your case, it'd be a corrupted island. The clouds are replaced with Demonite, and it's just in general corrupted, with a corrupted chest in it to let you get the corrupted item (Scourge of the Corruptor), if I recall correctly, you should be able to get the key needed from WoF, who I believe can drop both evil keys upon death.