Major spoilers! I stumbled upon these on Youtube and thought I'd share because it's pretty cool
You need to enable Debug/Creator mode to play Black Onion, hold down spacebar in the title screen and type "h-e-l-p-m-e", first video shows how to do it. I also linked a BO showcase from the same channel, they have a few more videos if you want to check them out
I did not discover these so I'm not responsible for any damage done to your save file, back up just in case!