The beginning somehow remind me of other anime, but i can't recall it.
The main protagonist got mail from unknown sender, and when they do the instruction, they got warped to other places.
And the imanity and races concept somehow not really that original, i have seen it on other manga/anime.
The anime is kind of lack on originality, but the concept of "there is no chances in game" and "where the victors already have been decided from start" it's really awesome