Horde Prime is the supreme ruler of the Horde Empire and only appears surrounded by a green smoke.
Horde Prime hails from Horde World, and travels the universe in his flagship called The Velvet Glove. It has been hinted that Horde Prime is Hordak's older brother because Prime's son, Prince Zed, calls Hordak his uncle and two UK She-Ra annuals in the mid-1980s state that Hordak is Prime's brother,.
Prime appears to be a giant 2 headed cyborg, and is immensely powerful. Prime has brought Hordak to his knees with just a thought. But it has never been revealed why.