Your friends keep telling you, "you can't play Arthas, man! Nobody's going to believe that your little human death knight is actually the Lich King in disguise. Get real!" But your idea just won't go away. You admit that creating a human death knight named "Ahrrthyss" might not be the best way to go about it, but you're in this guild which is devoted to fighting the Scourge, and you want Arthas to be a part of your story, not just an NPC who shows up in some quests and at the end of a raid.
We've already discussed a number of ways to be a villain in WoW – so you look at them to see if you can get one of them to work for you: The most obvious is to just start a new character and designate it to be one of your guild's antagonists, but the problem here is that making Arthas as an actual player character is way too Mary Sue. Such a tactic usually only works for very subtle villains (more like flawed heroes really), or for short-term possession, and your guild has done 3 "possessed by the Lich King's power" type stories already. You need something new! Another choice is to create a disposable villain, perhaps, some agent of the Lich King, which could be interesting, but still doesn't put you in touch with Arthas himself.
But there is another way, which many people have not thought of: to put the villain entirely in the shadows of the background, let him never actually be seen, but let his effects be felt based on what happens to the heroes. Arthas can indeed play a huge role in your story, without ever having to appear in person. It has been done to great effect before, even in novels. Sauron, anyone?