|This article is about the location in The Witcher 3: Wild Hunt. For the similarly sounding location in the comics, see Caed Dhu.|
Kaer Dhu is a ruined fortress situated in a valley across the lake from the abandoned village of Fornhala in central Ard Skellig of the Skellige Isles. It is just south past the swamp from Fayrlund and Sund and northwest from the road leading from Fyresdal.
The old castle contains the mastercrafted Griffin silver sword diagram and a nekker den filled with nekkers and possibly a cockatrice and some ulfhedinn prowling the area. It also contains a secret passage into some abandoned mine tunnels which exit on the other side of the mountain south of Fayrlund. These tunnels are also the site of the dead body of the alchemist, Klaus Kellerman, and the nekker he failed to catch after his employer, Sir Robert de Mere, paid and gave him the job to catch it. Sir Robert de Mere had planned to pay him to do so because he thought that the nekker was his missing son for wearing a tattered shirt with his son's monogram sewed on it.
- In the real world, Ker dhu means "Black Fort" or "Black City" in Cornish (Caer ddu in Welsh). Kaer or Kêr also means city in Breton.