Hana no Souretsu ~A Stray Children~

Stage: The Dita continent, while two-thirds of the earth are occupied with wilderness and monsters. A few small countries are standing in disarray across the continent. The countries are supervised by two major forces: The Marion Republic and The Setou Foundation.

The protagonist wakes up.
He was supposed to be dead.
He doesn't remember why.

He finds himself in a strange, dark and lonely underground facility...

[Translated from the official site]