Yes, that's who I'm talking about. I was kind of hoping there would be an "official/legit" way to battle that form again in the future, without resorting to Mods.
Not the new Dark Perpetua, but the normal one can be found in the Perpetua Rematch mod. It doesn't alter anything or change how the game is played, it just puts a level 100 Perpetua in the Forest as an option.