These creatures take on the a form similar to the Celestial that they serve. As an example the almost all of the Serephim that serve Tymora of the Waters appear as giant merfolk with long claws, fish bodies festooned with spines and shark’s teeth.
The Serephim are creatures used by most members of the Celestial Host to maneuver around the Vedic Concordance. Technically the Serephim are mortal. They are born, they live and they perish (after a very long time; some believe thousands of years). The Serephim act the enforcers for the Celestial Host. They are long lived, very intelligent and incredibly strong and tough. For a “mortal” to kill a Serephim; in nearly impossible. They are often huge, though not always and are devoutly loyal to the Celestial that created their race.