I have been playing Icecrown Glacier quests. A buddy told me about the quest OPPORTUNITY (given by Koltiras at Doom’s Hammer quest hub). You are sent to a hole beneath Icecrown Citadel that is said represents a strategical weakness to the citadel’s defenses (Think Star Wars’ Death Star fire-in-the-hole scene).
You end up blowing it up and falling down the hole into Azjol-Nerub’s outdoor Forgotten Depths. There you fight Faceless Ones, and find a boy’s ghost named Matthias Lehner (a play of words / anagram: Arthas Menethil). The Second video shows that you can play as Arthas the Lich King using Frostmourne to kill 200 Lordaeron footment, to rise them as ghouls to do your bidding. Instead of mana, Arthas consumes the scourge he rises to increase his powers. Matthias Lehner explains we are approaching Arthas the wrong way, for as killing hordes of undead, does nothing in the end as he rises more through the Alliance and Horde’s casualties.