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 Mek in Bozisha-Dar, but with the rise of the Sun-Lord dynasty the heir of house Mek flew the Dar 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 desparate tribes and clans of Harad. Suladân, a heir of the last kings of the Southern Confederacy came to unite both offices, the Kralji of Bozisha-Miraz and the Tarb of Sirayn 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.
The Black Serpent was interpreted as an epiphany of the dark Lord Sauron in his guise of Khaz-Gramaze.