History: Arthas was the son of King Terenas Menethil, ruler of Lordaeron. As a youth, he trained in the art of combat and joined the Silver Hand, a group of paladins. He became known for his exploits in battle. He also briefly dallied with a young mage named Jaina Proudmoore, but it didn't work out because long-distance things never end well. They decided to remain friends, which is as big a sign of impending doom as one can get.
Arthas and his trainer, Uther Lightbringer, were sent to the orc internment camps near Lordaeron to quell uprisings. Arthas defeated the orcs with the use of a magical fire weapon, but soon discovered that a plague of undeath was sweeping the northern villages of Lordaeron. The now-evil wizard Kel'Thuzad had transported the plague, originally the brainchild of the Lich King Ner'Zhul, to the towns of Brill and Stratholme through some infected grain. (Sorry for all the links to WoWWiki, but we're sort of joining the story in the middle here and all the names can get confusing.) Arthas and Jaina went on a totally platonic mission to stop the plague, which Arthas accomplished by killing Kel'Thuzad. Arthas then headed north to Stratholme to kill the demon Mal'Ganis.
When Arthas and Uther got to Stratholme, they discovered that the grain had already been distributed and the townspeople would soon become undead. Arthas ordered his troops to kill all the villagers. Horrified, Uther and Jaina refused and left with some of the Silver Hand forces. Arthas and his pals killed off the villagers and eventually ran into Mal'Ganis, who kind of just waved and said he'd see them later in Northrend. Gee, like that doesn't sound like a trap at all. Arthas and his followers blindly headed off to Northrend, where they met Arthas's old friend Muradin Bronzebeard. Arthas and Muradin searched the area for Mal'Ganis, but they were soon ordered to come back to Lordaeron by Terenas and Uther. Arthas said "Screw that!" and burned his boats so his men would have no way to get home but to kill Mal'Ganis.
Muradin had mentioned a mystical runeblade, Frostmourne, that would grant its wielder unlimited power. Arthas and Muradin figured that would be a nice thing to use against Mal'Ganis. Arthas discovered the sword in a fortress and, ignoring all Muradin's warnings about evil and corruption and bla bla bla, proclaimed he would "pay any price" to get the sword. Arthas wrenched Frostmourne out of the ice, killing Muradin in the process. Sword in hand, Arthas handily slaughtered Mal'Ganis's minions before confronting the demon. Mal'Ganis told him that the Lich King was speaking to him through the sword. Unfortunately for Mal'Ganis, the sword told Arthas to kill him. After defeating Mal'Ganis, Arthas fled into the wastelands of the north alone.
Months later, Arthas came back to Lordaeron and was hailed as a hero. But as he knelt at the throne of his father, Arthas drew Frostmourne and killed Terenas. He had lost his soul and been corrupted by Frostmourne. Arthas deicded to reanimate the body of Kel'Thuzad to serve the Lich King once more, killing Uther along the way. In the process, he invaded the city of Quel'Thalas and killed its ranger-general, Sylvanas Windrunner. (If that name's familiar, it should be.) He then raised Sylvanas from the dead as a form of revenge, which seems pretty petty for a guy with unlimited power. Kel'Thuzad was also raised from the dead, and he and Arthas went to Dalaran to talk with Archimonde, the general of the Burning Legion's forces. After a lot of fighting, Arthas, Kel'Thuzad, and the Scourge successfully summoned Archimonde into Azeroth. Archimonde then proclaimed that the Lich King was of no further use to the Legion. Looks like someone picked the wrong side!
Arthas and Kel'Thuzad laid low for a few months. Then Arthas traveled to Felwood to fight the still-demon-hunter-not-yet-actual-demon Illidan. The two fought to a standstill, and Arthas suggested that Illidan consume an ancient warlock's skull to give himself the power to fight the demon Tichondrius. Illidan, never one of the world's greatest thinkers, agreed. Then Arthas went ... somewhere else for a few months. Maybe he was taking a bike tour through Europe, I don't know. Anyway, Illidan killed Tichondrius, and several months later Arthas went back to Lordaeron and took it over from Tichondrius's followers. There, with Kel'Thuzad and Sylvanas at his side, he finally ruled his kingdom.
However, Ner'Zhul, the Lich King, was ailing. His powers were weak enough that Sylvanas slipped out of his control. Sylvanas set up a trap for Arthas and shot him with a paralysing arrow so she could torture him and whatnot. Kel'Thuzad stepped in and saved Arthas, who was then called to Northrend by the Lich King. Unfortunately for Arthas, Northrend was currently being invaded by Illidan and Kael'Thas, who were under orders to kill the Lich King for the Burning Legion. (Yes, the Scourge and the Burning Legion and the Illidari and the Alliance and the Horde all hate each other. Try to keep up.)
Arthas fought through a bunch of mobs to try to get to the Frozen Throne, where the Lich King sat. It was kind of like the General Rajaxx event in AQ20, except the mobs didn't bug out every other try and the loot presumably didn't suck. He was joined by a new ally, the spider lord Anub'arak, and together they defeated naga, blood elves, dwarves, and even some sort of ancient evil called the Forgotten One. They activated the obelisks and opened the door to the Frozen Throne, only to run into Illidan, who was lurking around the glacier's base. Illidan and Arthas then fought each other one on one for NO RAISIN (ten points if you get the reference.) Seriously, I don't know why they didn't just let their armies fight it out while they sat in the back and sipped on daquiris. Arthas sliced Illidan's chest open with Frostmourne, and left him "dead" in the snow. Of course, this is Warcraft Dead, meaning "alive." Arthas ascended to the Frozen Throne and found a suit of armor sitting there. The voice of Ner'Zhul told him to return Frostmourne to the Frozen Throne. Arthas struck the Throne with his sword, shattering it. Ner'Zhul's spirit was released from its prison, and when Arthas put Ner'Zhul's crown on his head, the two beings fused into one entity. Arthas was the true Lich King.
Where he is now: Arthas/Ner'Zhul/the Lich King now rules Northrend, where he's built himself a giant fortress. Anub'Arak rules Azjol-Nerub as one of his lieutenants. Kel'Thuzad directs the Scourge in the Plaguelands when he's not busy being farmed for Nexus Crystals by a handful of guilds. Arthas, himself, is not really doing anything to hurt Azeroth, and he doesn't even seem to have a prescence in the Outlands. His voice is heard briefly in Naxxramas, as he gives instructions to Kel'Thuzad.
Identifying characteristics: Arthas speaks with the combination of his human voice and the voice of Ner'Zhul. Blizz's lore people have said that the Lich King is the fusion of Arthas's personality and body with Ner'Zhul's powers, which means there could possibly be some Tyler Durden-like multiple personality thing going on. He wields the soul-stealing sword Frostmourne and wears Ner'Zhul's armor, which features skulls as a prominent design theme. He has long, scraggly blond hair and is probably suffering from frostbite.
Why we should probably do something about him: Everyone seems to have forgotten Arthas in all the "Yay, let's go fight the Illidari and the Burning Legion and everyone" craze. He probably craves attention and hates being ignored. Actually, the smart thing for the Horde and Alliance to do would be to ally with one of their three villain groups to defeat the other two and then turn on their last enemy, but the Alliance would probably start complaining about honor and the Light and morality, etc., etc. So instead we get five factions that hate each other.
For more information: WoWWiki on Arthas, World of Warcraft Story Page on Arthas, Arthas betrays Terenas, Arthas becomes the Lich King.
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