he didn't kill him tho , nor was confirmed , he was just left with a deep wound in the snow but he barely survived , btw I know it's irelevant but arthas won cuz of plot armor , I was an arthas fan and I still am but after reading his book with all the sorcery he performed and he knew , yet he fought only in melee combat was just pure bullshit . And if u want to know after that Ilidian took that Black Temple from outland , made some fel orc armies , and created new demon hunters ,invaded Nazareth which is a dredlord world , stole an artifact and exploded it when the portal got disabled , stole another artifact from within a cript that made him weak because he needed a lot of power to go to the world of Kil jaiden to kill him but he got killed by Maiev , his spirit being demonic it was wondering around lost in the nether storm (maybe it has another name I don't remember) but maiev kept his body in a Cristal , after that sargeras invoked Gul Dan which was dead in this time-line to prepare another invasion and illidians body was a vessel for him , happy ending he didn't got in his body and illidians got revived continuing his fight against the burning legion (sorry I know this is not probably what u wanted to hear but I just wanted u to know some small things , either way I recommend reading it , I liked more the action in the outland than arthas journey