The Black Serpent was the symbol of the great southern High King of Haradwaith, the Serpent-Lord, and the great southern army or Serpent-Army.
Originally it had been the heraldic sign of the House of Mêk in Bôzisha-Dâr, but with the rise of the Sun-Lord dynasty, the heir of House Mek fled the Dâr and hid among the Sand-Men. It was a chieftain named Mardat of Kârna who first claimed to be the rightful Serpent-lord, and after his death the legend of a king to return and unite the Haradrim prevailed among the desperate tribes and clans of Harad. Suladân, an heir of the last kings of the Southern Confederacy, came to unite both offices, the Kralyî of Bôzisha-Mîraz and the Taryb of Sîrayn, and became the first true high king who claimed to be the returned Serpent-lord. With him the ancient banner of House Mek came to replace the older symbol of the Southern Dragon of the former Army of the Southern Dragon. After he had formed the Reunited Kingdom, King Aragorn Elessar had to face the Serpent-lord's Diadochi, local uprising chieftains, until Haradwaith could finally become a pacified province.
The Black Serpent was often interpreted as an epiphany of the Dark Lord Sauron in his guise of Khäz-gramaze though it may ultimately have derived from the signet of Kathôl a Second Age ancestor of House Mêk.