A high-ranking demon from the Phenex family—one of Hell's most prestigious bloodlines—Raiser was supposed to marry Rias Gremory as part of an arranged marriage between their two houses. He's the third son of the family, which already gives you a sense of his standing in the demonic hierarchy.
When Issei found out about the engagement and objected, he challenged Raiser to a fight. It didn't go well. Raiser casually had one of his Pawns take Issei out in a single hit, which pretty much established where things stood power-wise.
Rias, understandably upset both about the arranged marriage and what happened to Issei, proposed settling everything through a Rating Game instead. This turned into the actual confrontation—a formal supernatural battle that would determine whether she had to go through with the marriage or could refuse him. It was her way of getting revenge and taking back control of her own life.
Content compiled by AnimeList.moe from publicly available sources.
