Betrayed, I Met The Demon Lord

Van and his friend Magus had reincarnated in another world with magic. Tasked to kill the demon lord, they set forth. Van was granted very rough skills that made life for him in that world a living hell, yet he muscled through. Magus however, was bestowed with unfair skills. Due to some of said skills, he gained a harem, which in time, included the girlfriend Van made in the world. After beating the Demon Lord, and after finding out his girlfriend had been stolen from him, he got very upset and so he went to propose to... The Demon Lord...!?