Oh oh my turn! my turn!
In Wc3 when Arthas first gets Frostmourne he wields it like a 2handed sword, he only one hands it when he gets on his horse. This horse also rears itself in sync with Arthas so he can get extra velocity for his strikes.
In Hots the Lich King uses 2 handed animations with Frostmourne.
In OG Wotlk there was an artifact level item called Frostmourne, it was a 2handed sword. This was before artifact weapons were accessible to players. Here is a link:
https://wowpedia.fandom.com/wiki/Frostmourne_(item)It was later changed into a Grey Item but still retains the model and the whole being a 2handed sword.