Datamining points to a mysterious Frostmourne model in Zereth Mortis, near the Sepulcher of the First Ones raid. This may be a fun Easter Egg, only intended to be visible in early data and removed by 9.2 launch, for several reasons: it's located in an inaccessible outdoor area by the raid instead of Zereth Mortis proper, and Frostmourne was shattered at the end of Wrath of the Lich King, reforged into the Blades of the Fallen Prince artifact at the start of Legion.
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