"Do you really wish to know?" — Spoilers from the books and/or adaptations to follow!
Havart var Moehoen served in the Nilfgaardian army as Field marshal after the death of Menno Coehoorn. He was in a position of confidence with Emperor Emhyr var Emreis, and so was granted governorship over Temeria.
We can see him for the first time and only time when we come to Crow's Perch in Velen, where he is making deal with the Bloody Baron but leaves when Geralt of Rivia arrives. After this he is not shown anymore, only mentioned in a letter.