Originally Posted by
Magnagarde
It failed because they told a very specific piece in the story that newcomers to the universe can't understand, while presenting it in a way in which it didn't happen canonically, alienating fans too.
Had they made a movie about Arthas or the Burning Legion itself, it would've done a lot better in my opinion. Everyone can understand who a prince is meant to represent, everyone can understand a grain plague ripping through a kingdom and everyone can understand the concepts of undeath, holy knights, retribution and so on.
I'd personally show scenes of KJ making Ner'zhul's armor and trapping his soul inside it, hurling the iceblock made in the TN onto Azeroth. Fast-forward to Arthas discovering the plague and heading to Northrend to kill Mal'ganis, picking up Frostmourne and ending the movie with Arthas killing the king and the bells ringing as the ending scene.