Nifflevar is a vrykul town, held by the Dragonflayer clan, located on the cliffs above Daggercap Bay in the Howling Fjord. They will shoot ships coming through the pass using harpoons on chains.
The cliffs of Niffevar from the BlizzCon 2007 press CD.
Nifflevar concept art
They don't actively attack ships in-game.
- The name Nifflevar is likely a reference to Niflheim, the "mist home", one of the realms in Nordic mythology.