Bran Mak Morn is a fictional barbarian hero created by Robert E. Howard. He is the last king of the Cruithni Picts. The character lived in Roman times during the period when Rome had invaded but not entirely subdued the southern British Isles.
Following Bran Mak Morn's death, he is worshipped as the demigod the "Dark Man" or "Dark One" by Scottish and Irish tribesmen of Pictish descent. In Howard's story "The Children of the Night" (1931) set in then-contemporary times, a "Bran Cult" is thought to survive among relict populations descended from the Picts.
Connection to the Cthulhu Mythos
Bran Mak Morn is central to the shared universe in a way that Conan (in the original writings by his creator) never was. Bran and his adventures take place in the shared universe of the Cthulhu Mythos as it was being developed by members of the Lovecraft Circle and Lovecraft himself.
In the Howard-penned Bran story Worms of the Earth, explicit mention is made of black gods of R'lyeh, resting place of Cthulhu. In the earliest known draft of Worms, Cthulhu himself is mentioned but this was later changed to Nameless Gods in the final version. Also mentioned is Dagon.
Lovecraft in turn references "Bran" in his story "The Whisperer in Darkness", which is interpreted by Clark Ashton Smith to be either Bran Mak Morn or possibly Brân the Blessed of Celtic mythos. (CIRCLE: Selected Letters of Clark Ashton Smith #209)