In mission 10 Arthas become insane an betray his men. It i a dark page, torn from the adventure book. And now it is time for him to do what exactly demons planet, to take the cursed sward of Frostmourne. After Arthas take the sward, his attack become chaos and not normal. Now its time to ride the Mal'Ganis base, and unfortunately for him, his master have double cross him, allowing Arthas to take his revenge.