Just watch a movie about arthas....
In warcraft 3,Arthas have done lots of evil doings,Arthas is actually the "old version"
of omniknight which is A.K.A LoA,lord of averus.
As soon as he hits the frozen throne,he is now a part of scourage.Wearing that helmet,makes him invincible!Controls scourage's army. Please watch =)