Soul Strangler was a mysterious but frightening villain that acted as one of the main antagonists of Black Panther Vs The Klan - a special 10-part story revolving around Marvel's premier black superhero and his confrontation with the real-life hate-group, theKu Klux Klan.
Soul Strangler came as the more supernatural side of the story (perhaps to add more threat to the Black Panther - who could easily outmatch the non-powered Klan) - yet his origins were never fully explored, he claimed to be the embodiment of the Klan's collective hatred made flesh but he could of easily have been the ghost of a former Klansman (or former victim - see comments below) or a demon such as Mephisto playing on the KKK motive for their own wickedness, another plausible explaination could be that the Soul Strangler was an avatar of Master Hate himself.
Regardless of the demon's origin the Soul Strangler fought against the Black Panther numerous times throughout the storyarc until he was finally overpowered and beaten up, as Black Panther beat the evil spirit it faded away, though it is unlikely a supernatural being such as the Soul Strangler could permanently die (though due to his appearance being limited to this series it is unlikely he will return)
As stated above the Soul Strangler remains a very enigmatic being and the reader is left to make their own choices on just who (or what) the Soul Strangler is - it is also worthwhile to note that despite being tied to the KKK the Soul Strangler appeared to have "dusky" skin, this leads some to believe that he could be the vengeful spirit of one of the Klan's victims - others believe it to be an embodiment of the Klan itself, perhaps the truth of the matter is we will never really know the truth.. (unless Marvel ever revisits the series, which seems unlikely).
Also in another unusual twist the Soul Strangler's robe is red - while the KKK robes are traditionally white - as with many things about this character the reason for this difference is not explained.
The skin coloration may be explained in the story itself as the group Panther fought were actually the Dragon Circle, a KKK-inspired group who (for some reason) were racially diverse despite its founder being a former High Wizard of the Klan.