one of my first runs I made it to that point with apollyon and I did the same thing, I even remember seeing the door right there but it didn't even register in my mind as anything relevant because why on earth would they put the new area right after that stupid thing
i have a way to the void after fetus fight now?

e: actually wonnered again, had chaos and got missing no
rolled in to cursed eye with short ranged firemind
but i got 70 something scattering hot bomber boy bombs, curse of the tower and and the remote detonator
so i was just able to run around the room laying bombs, backing into a corner and loving him up and if i got hit it would spawn more bombs thanks to the curse
got really close, was down to half a red heart and out of bombs at the end, popped a mystery pill that was infested and he got forgeted by the spiders that spawned off poops he had made