You know where this is going by now.

A DOOM X GATE: Thus the JSDF Fought There Tale.

A lone man, one long forgotten by all, returns to his home planet, only to see nothing but death and abandonment. Realizing the world needed him once more, he roams the Earth, finding others with the same intention as his along the way.

It's a pretty good story about going into another world and the mc became a skeleton in a dungeon. what mystical adventures will he have.