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Fighting Mega Satan on hard mode. I didn't plan to do this, I just wanted to beat Blue Baby on hard with Azazel to get the items. First item room I went in to was ludo, then I got sacred heart and realized I could do something good with this run. I didn't get any unlocks or achievements for doing this, and there's no icon for it on the character select even though it wasn't seeded, but I did get ending 16 and the icon for my save file changed to a dead Satan head. If you want to try it, it's pretty easy, the seed is AAX2 8KDZ.