One of the voodoo loas of Haitian mythology. He is one of the more dreaded loas, because he represents death. He is a guardian of the crossroads, where our world intersects with the spirit world, and he is often depicted as a skeletal figure smoking a cigar, drinking rum, and wearing black clothing, a black top hat, and dark sunglasses.
Though he's often depicted in Western media as a demonic supervillain of sorts, those who follow vodou don't think of him as a pure negative force. As the guardian of the lands of the dead, Samedi helps to escort the dying into the next world, so ceremonies will call on him on behalf of those who are dying. He's also the loa of resurrection, so he's sometimes called upon by those who are gravely ill.
François "Papa Doc" Duvalier, the sadistic former President-for-Life of Haiti, modeled his own appearances on Samedi, which probably does a lot to explain why the loa is considered a pure villain in the West.
Samedi has a morbid sense of humor and is closely associated with Baron Cimitere and Baron La Croix. When he rides (or possesses) a worshipper, they can be identified because they like to smoke cigars and wear sunglasses and dark clothing.
In English, his name is Baron Saturday, which is a pretty awesome name, too.