"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
Morteisen was a Nilfgaardian officer and commander of the Morteisen Battle Group, one of three main divisions of the 7th Daerlan Brigade. He reported to Elan Trahe.
He met his end in the Battle for the Bridge on the Yaruga river when a dwarf volunteer in the Lyrian Army ran him though. He and Cahir had just recognized each other during battle and this shock was likely what caused him to lose focus and be killed.