This porn video tag refers to a hentai (Japanese for "perverted") video featuring the characters Rias and Gremory. Hentai usually involves animated adult content, specifically focusing on detailed sexual scenarios involving these characters from the source material. In this case, Rias and Gremory are likely characters in a manga, anime, or other Japanese media, where they engage in explicit sexual activities with each other or with other characters. This tag is for an adult audience familiar with the source material and its characters, as it describes explicit content not suitable for minors.