This game actually takes place hundreds and thousands of years after an Earth post-apocalyptic scenario considering the overgrowth and ruins and literally Hell raising from the core of the planet except the demons are office workers and the dead just don't like people waking them up

there was a massive virus that turned the earth's population into slime and the slime seeped into the underground labyrinthine clockwork that comprised most of the pre-apocalypse earth's infrastructure which is why we see slime everywhere

and the Core is the last pure sample of that virus

we're going to doom ourselves into becoming slimes

that's the ending to spiral knights have fun everybody