Question: What undead creature from Norse mythology have two Royal Norwegian Navy ships been named for?
Context: "Skjold"-class corvettes (skjold means "shield" in Norwegian) are a class of six large, superfast, stealth missile corvettes in service with the Royal Norwegian Navy.  The boats were formerly classed as MTBs (motor torpedo boats) but, from 2009, the Royal Norwegian Navy has described them as corvettes ("korvett") because their seaworthiness is seen as comparable to corvettes, and because they do not carry torpedoes.  They were built at the Umoe Mandal yard.  With a maximum speed of 60 knot , the "Skold"-class corvettes were the fastest combat ships afloat at the time of their introduction. Two ships of the Royal Norwegian Navy have borne the name HNoMS "Draug", after the sea revenant Draugr: HNoMS Valkyrien, HNoMS Valkyrjen (in Norwegian the prefix KNM is used) has been the name of several ships in the Royal Norwegian Navy.  Most of them have been in some way connected to the Royal Norwegian Navy's torpedo boats.  The name honours the valkyries of Norse mythology.  It has also been said that the ships have been given this name in honour of all women.  Some of the ships: Two Royal Norwegian Navy ships and a class of patrol boats have been named Rapp, meaning "quick". Two ships of the Royal Norwegian Navy have borne the name HNoMS "Odin", after Odin, the primary god in Norse mythology: Several ships of the Royal Norwegian Navy have been named Æger, after Ægir – the Jötunn king of the sea in Norse mythology: HNoMS "Kjell" was the final ship of twenty-seven 2nd class torpedo boats built for the Royal Norwegian Navy, launched at the Royal Norwegian Navy's shipyard in Horten on 12 March 1912 with build number 106.  "Kjell" saw more than 32 years of service, the first 28 years in the Royal Norwegian Navy during the First World War and in the interwar period, the last four in the "Kriegsmarine", having been captured in the first days of the 1940 Norwegian Campaign.  After being rebuilt as a minesweeper by the Germans, she was sunk by Royal Air Force de Havilland Mosquito fighter bombers on 28 September 1944.  Divers rediscovered the shipwreck in 2006. The draugr or draug (Old Norse: "draugr" , plural draugar ; modern Icelandic: "draugur" , Faroese: "dreygur" and Danish, Swedish, and Norwegian: "draug" ), also called aptrganga or aptrgangr , literally "again-walker" (Icelandic: "afturganga" ) is an undead creature from Norse mythology. The Marinens Flyvebaatfabrikk M.F.6 was a two-seat biplane floatplane built by the Royal Norwegian Navy Air Service aircraft factory Marinens Flyvebaatfabrikk in 1921.  The M.F.6 was designed and employed as a trainer aircraft.  The type was the last pusher configuration aircraft in service with the Royal Norwegian Navy Air Service.  The M.F.6 functioned as a makeshift solution until a more modern tractor configuration aircraft could be built.  The type was retired once a more permanent solution to the Royal Norwegian Navy Air Service's training needs was found in mid-1920s. HNoMS "Sæl" was the penultimate vessel of the ten 1.  class torpedo boats of the Royal Norwegian Navy.  She was built at the Royal Norwegian Navy Shipyard in Horten in 1901, with yard number 85.  She was to see close to 40 years service with the Royal Norwegian Navy, taking part in the preparations for war in connection with the dissolution the union with Sweden in 1905, enforcing Norwegian neutrality during the First World War and opposing the German invasion of Norway in 1940.  She was lost in battle with "Kriegsmarine" vessels at Ånuglo in the Hardangerfjord on 18 April 1940.
Answer: Draugr