Xarthisius was a Nilfgaardian mage of dubious note, more of a fortune-teller and astrologer at the court of Emperor Emhyr var Emreis. When the future emperor was still in his youth, the mage convinced him that a cure for his curse could be found in the North. The mage was later rewarded by the emperor with his own tower and astrological equipment.
Later, he given the task of locating Ciri by the Emperor. The area he identified proved to be a wasteland in the Tir Tochair mountains where Stefan Skellen found nothing but scorpions and vultures. For this and other miscalculations, he was unceremoniously tossed in the dungeon. He is also responsible for locating the "false Ciri".