No Game No Life, Vol. 5 (Light Novel)

In a world where games decide everything--even national borders--a genius brother-sister duo aim to put their gamesmanship to the test...to save humanity!
In the world of Disboard, everything is decided by games. Since arriving in this strange place, genius gamer siblings Sora and Shiro have risen to become king and queen of what's left of Disboard's humans. Their latest challenge is winning an unwinnable romance game against the races of Dhampir and Siren. To uncover the true strategy to beat this sadistic game of love, they head for the home of the angelic Fl?gel: the midair city of Avant Heim. But the Fl?gel are a hideously powerful race, created specifically to kill gods. Will things really go as planned?