/|\\ Swarm intelligence of Twitter, I need urgent help.

I‘m looking for an anime I am emotionally attached to, due to circumstances of when I first viewed it.

I can‘t find it.

It‘s about a girl or boy(?) finding a hidden underground door that leads to a hidden world.

/|\\ Memory update:

There was a scene where a character was in the ruins of a colloseum-like place.
Then the sun started setting and creatures who could only move in shadows attacked.
He/she fled upwards until passing out and being rescued by the natives of that world/place