You obviously haven’t read the book. The Lich King spent years entombed in ice on the Frozen Throne. During that time there was a three corner battle for Arthas’ soul going on between the remnant of Ner’zhul, Arthas’ core personality, and the last remnants of his Humanity.
That opening scene in the cinematic where the first thing you see is the Lich King opening his eyes? That’s the moment in the Arthas novel where Arthas has eliminated both Ner’zhul’s presence and that of his remaining humanity. Everything that occurs after that, the whole Xandadu plan of creating a new force of uber agents by orchestrating the entire series of events that occur duringt he expac… is under Arthas’ agency alone.