The Outer Dark are one or many extradimensional realms, allegedly ruled by the
Haemovore Kings of the Outer Dark, and seemingly part of the Splinter Realms.
References to the "Outer Dark" would typically refer to
demons of the third class, of extradimensional origin, primarily the Old Ones and the N'Garai.
Set and Ibis fought in the Outer Darkness for a "thousand centuries", at least since and throughout the  Pre-Cataclysmic Age, and into the Hyborian Age.
Hyborian Age, the "devils from the Outer Dark" were not uncommon, lurking "outside the belt of light which surrounds" Earth, some finding their way there and taking flesh forms, according to the Wise Men of Zamora.
In modern days, the Outer Dark was allegedly ruled by
Haemovore Kings of the Outer Dark, part of the Lords of the Splinter Realms. 
Residents Haemovore Kings of the Outer Dark; Demons of the third class, including Old Ones; N'Garai
Set and Ibis fought in the Outer Darkness. 
The Outer Dark is a recurrent "location" mentioned in the
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