Demawend was the son of Virfuril, king and ruler of Aedirn. For many years, he participated in an ongoing dispute with Henselt over the North March. As a ruler he was known for being rather cruel and envious, but also quite intelligent and knowledgeable about the tactics of war. He was, for example, able to predict Emhyr var Emreis' moves during the war with Nilfgaard. He showed little or no regard for the pain or suffering of others and had a particular hatred of elves.
Yennefer used to sit on his council and Rayla was an officer of his special forces. After the war, he was one of the negotiators of the peace treaty.
In The Witcher 2: Assassins of Kings
Early in the game, a mysterious individual claims to have killed Demawend, and to have his head as proof. It is unknown whether 'Kingslayer' is being truthful in this claim but it is quite likely that is the case as the cutscene which has been released showing this shows a crown near the severed head.
cs:Demawend