Lord Arrgthas is a reference to Arthas from Warcraft III and World of Warcraft.
He is difficult to defeat due to his high life and damage reduction effect.
He drops a Permafrost Scimitar, variants of which can be purchased from a shop that appears after his defeat.
|He is not a lich king just yet, but he has armor and a very cool looking sword... that you might be able to take from him!|
Lord Arrgthas can be fought at level 10.