Karkut appears to be a tuskarr deity who is also known as "He Who Watches Over The Dead". According to Ataika, death knights such as Thassarian bear his mark. Karkut is said to watch over fallen Tuskarr so they do not lose their way. The Tuskarr believe that Karkut is currently walking through the land, collecting the souls of the dead.
It is speculated that the tuskarr believe that the Lich King is Karkut, as undeath is under the Lich King's dominion in Northrend. This would fit in with the suggestion that the death knights bear his mark and that Karkut now walks the land collecting souls. On the other hand, the tuskarr also seem to believe that Karkut looks favorably upon them, which would make little sense if they thought he was the Lich King.