According to the Deeds of the Dying Person

A matter pertaining to the angel of death is that when he comes to take the soul of any person he assumes a face according to the deeds of the dying person. It is mentioned in narrations that once Ibrahim (a.s.) asked the angel of death, "What shape do you assume while fetching the soul of unbeliever?" The angel replied, "You will not be able to look at that frightening shape." The Friend of God repeated, "I intend to see it." He saw the angel in the form of a black man. His hair was standing on its end all over his body. A foul stench emanated from that frightening figure. His clothes were dark and flames and smoke leapt out from his mouth and nose. Ibrahim (a.s.) became unconscious. When he came to sense he said, "Even if the unbeliever is not given any other punishment, seeing you in this form is torture enough for him."

On the other hand the shape in which the angel of death appears before a believer has been mentioned in a number of traditions