Best of the best

that guy was summoned and enslaved by a saintess in an other world to protect the hero and defeat the demon lord. he uses dark magic (the magic from monsters). he asks to go back home, but finds out that dungons opened on earth at the same time he was kidnapped (that bitch unleashed monsters there). with no dark magic, he climbs to the top of the military to fight magic monsters and gain his powers again!