Nekir, the Black Guardian, is one of the twin Abyssal lords who guard the Gates of Heaven on the 77th Layer of the+Abyss. These gates are said to be the back door to the Upper Planes.
Nekir's twin is Munkir, the White Guardian. Long ago, Nekir and Munkir were battled by Azor'alq.
Sources:
Fiendish Codex I, page 155
Living Greyhawk Journal no. 3
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