At some point before squaring off against P.R.O.B.E., the Greth attempted to invade Ertshea, later claiming the title of "Scourge of the Wizards of Ertshea". (PROSE: Out of the Box)
Behind the scenes[]
The name of Ertshea is reminiscent of Earthsea, the setting of Ursula Le Guin's highly influential book series of the same name, the first book of which was entitled A Wizard of Earthsea.