King Hiss action figure from the Masters of the Universe original series. King Hiss is the leader of the Snake Men and never appears in the original animated series.

King Hiss as he appeared in the Masters of the Universe mini comics

An ancient wizard and leader of the Snake Men, King Hiss had once been an intergalactic conqueror whose empire had spread throughout the universe. Eons before the time of He-Man, King Hiss terrorized Preternia from his stronghold in Snake Mountain. He battled the ancient warrior-wizard He-Ro and served the Unnamed One. King Hiss was defeated when the Council of Elders banished him and his army to a dimensional Void beneath Snake Mountain.

In He-Man’s time, Skeletor inadvertently set King Hiss free when he discovered the pool of power beneath Snake Mountain. King Hiss then tried to free his army from the Void but only managed to summon Rattlor and Tung Lashor, who were working for the Horde at the time. Later, when King Hiss entered Viper Tower, he was able to summon more Snake Men to join his ranks. King Hiss formed an uneasy alliance with Skeletor, each intending to betray the other once they’d vanquished their mutual enemies. Though usually appearing as a heroic-looking man, King Hiss’s human guise could melt away to reveal a mass of writhing snakes in humanoid form. In addition to his magic, he had a hypnotic gaze to mesmerize his foes. King Hiss displayed hostile feelings toward Hordak which dated back to ancient times. Upon learning that Rattlor and Tung Lashor had joined the Horde, Hiss remarked, “The Evil Horde still exists? And you dared to serve Hordak?”

Wiki Grayskull History

Early War for Dominance

According to the Wiki Grayskull, despite his defeat at the hands of Hordak and the death of King Grayskull, King Hiss and his Snakemen returned once more in order to conquer Eternia. This time though they would face the might of the Elders and Zodak. The leader of the Snakemen would unleash the gigantic snakeform of Snake Mountain on his enemies but this brutish monster was defeated by the power of the Elders who in turn imprisoned Hiss along with his army in a timeless dimension called the Void. The entrance of this realm was deep within Snake Mountain and required a special key in order to free the denizens trapped within.

Freedom from the Void

Centuries later, King Hiss would nearly escape from the Void due the machinations of one of the Snakemen’s descendants known as Kobra Khan. However, this failed but allowed Rattlor to escape the timeless prison in order to plot Hiss’s freedom once more. This time, they would succeed and King Hiss would take his army and reclaim Snake Mountain after defeating Skeletor and his Evil Warriors effortlessly. He would almost devour Evil-Lyn who had allied herself with King Hiss but once she promised the power of the Elders to King Hiss, he spared her life on the condition that he take her to where their power was held namely in Castle Grayskull.