You can get it from Heroes of the Storm directly using a CASC editor and it will work in StarCraft II since version 3.0 which synchronized the game engines. Just import it, its extension file with all the animations and the textures and it will work in StarCraft II. Blizzard has even given people the thumbs up to do this as long as you do not try to recreate Heroes of the Storm in StarCraft II.
If you are actually trying to request that someone port the Heroes of the Storm Arthas model to Warcraft III as a mdx file then you are not asking very clearly. It also is not directly possible as it uses features such as normal maps, emissive and physics which are not supported in Warcraft III. Let us not forget the obvious that Warcraft III already has an Arthas model, and HiveWorkshop hosts many which have had Lich King armor added to them.