"So, you've taken up Frostmourne," Mal'Ganis mused to the young prince, visibly impressed with the young human's courage, "at the expense of your comrade's life...just as the Dark Lord said you would. You're stronger than I thought."
Arthas looked on the dreadlord with total disdain; the whispering in his head growing louder and louder the longer he held the blade. The voices were trying to drive him mad, but like at the moment he looked on Muradin's corpse after he claimed the runeblade, Arthas just didn't care. Frostmourne's whispering was another annoyance, like the troops that had fallen to Mal'Ganis's undead soldiers.
"You waste your breath, Mal'Ganis," Arthas said unemotionally, locking eyes with the demon, "I heed only the voice of Frostmourne now."
Mal'Ganis's bloodstained mouth pulled up into a grin. "You hear the voice of the Dark Lord. He whispers to you," Mal'Ganis looked pointedly at the runeblade, "through the blade you wield. What does he say, young human? What does the Dark Lord of the Dead tell you now?" Mal'Ganis was confident now; the human would not dare harm him against the Lich King's wishes.
Arthas looked down at the savage blade in his hands, the runes glowing as the whispers finally began to make sense. As the Lich King's whispers penetrated deep into the young prince's mind, Arthas looked back up at the dreadlord, and smiled. "He tells me," Arthas said, emphasising the words he spoke, "that the time for my vengeance has come."
"What?" Mal'Ganis asked, so surprised he didn't make a move as Arthas charged at him, the blade pointed at his chest, "He can't possibly mean to-" Mal'Ganis's words were cut off as Frostmourne slid into his chest, all his demonic power fading into the blade. His body began to burn, and the dreadlord had one last chance to cry out before his body disintegrated into dust.
"It is finished," Arthas said, a small part of him wondering why he didn't care that his vengeance had been finally carried out. He had defeated the one that had brought plague and death to his homeland, turned his people into living corpses, and he felt no joy at bringing him down. Why?
'Kill them all,' the Lich King said, 'the entire human camp. Kill them, and use my power to erect your own army. Do this, and a Death Knight I will make you. Then, you can claim your homeland for your own, as its rightful king.'
Arthas looked again at the blade in his hand, raising it so that the blade pointed to the sky. The runes seemed to be glowing now, as if Mal'Ganis's power was pleasing to the blade. Demon blood was still on the blade, and Arthas raised his free hand to touch it. But, even though he was the blade's wielder, the metal was colder than even the howling wind that tore through Northrend.
"Frostmourne indeed," Arthas said, turning to the warriors that had helped him raze the undead base to the ground. Footmen and knights alike were lying wounded, being tended to by the elven priests, the holy Light glowing brightly in the frozen wastes that surrounded them. Arthas's Captain approached him, still concerned for his leader.
"Milord," he said tentatively, "Now that Mal'Ganis is dead, will you keep your promise and return us home?"
"Of course," Arthas said, "I promised, didn't I?" He was still holding Frostmourne aloft, and he stared at the blade, enraptured by its powers. As he made the decision to follow the Lich King, the priests and sorceresses looked at the prince in horror. Tied to magics as they were, they could sense the prince's rejection of the Light, and the embracing of Darkness. Arthas raised the blade and removed the Captain's head from his neck, fresh-blood soaking the still-wet ground. The elves gathered their magics and attacked Arthas, who simply grinned and slashed Frostmourne through the air, casting his own spell. He negated their magics, and advanced on the horrified elves, a maniacal grin playing on his features. Frostmourne flashed in the morning sun, ripping elves and humans alike open, devouring their souls greedily, adding to the already fearsome power of the runeblade. After dealing with his remaining troops, Arthas raised the blade high above his head.
"By the power of the Lich King," Arthas intoned, the sky beginning to blacken as he invoked the unholy powers of the blade, "I recall you from your slumber, to serve in his name. Rise, fallen warriors, and fight for me. Rise again, and serve the mighty Army of the Undead!" Thunder roared as the blade glowed. Lightning flashed the sky as Arthas did what no necromancer could hope to accomplish, not even the mighty Kel'Thuzad. He didn't raise the bones of these corpses; he didn't mutate them into mindless zombies; he infused the dead with the power of Frostmourne, the power of the Lich King, and brought the corpses back themselves, raising them like puppets, jerky and totally subservient. With his troops back at his side, Arthas began his descent to the camp, where the unsuspecting humans would see him and his troops returning triumphantly. But, after a moment, they would take in the lack of cheering and revelation in their victory, and notice that some of the troops were missing limbs, and that Arthas's loyal captain was missing his head. As they approached the camp, sounding of the work of mining and lumbering, Arthas would raise Frostmourne high above his head, sunlight reflecting of the cursed blade.
And the undead warriors would begin to chant; "All hail the Lich King; All hail King Arthas," in a dead monotone. And like the wave of undead that had broken upon the camp just hours earlier, Arthas broke upon the souls that still dwelled there. Even Frostmourne's everlasting hunger was partially sated by the souls Arthas reaped. As he did, Arthas began to laugh as the camp began to burn to the ground. The holy aura of the paladin had already faded, and the tainted aura of a Death Knight began to shine. After the camp had fallen; it hadn't even put up an adequate defence, Arthas left the undead he had risen there, and advanced into the driving snows of Northrend, the cold not even affecting him. Arthas dropped the book of holy spells, as uncaringly as he had his hammer, and made his way slowly through the snow, the cold unrelenting wind chilling his skin about as much as a cool lake on a summer's day. No amount of cold weather could affect him now; Frostmourne's power would make sure of that.
On that day, Arthas, crown prince of Lordaeron ceased to exist. Now, he was a servant of the Lich King, a leader of the undead armies and the Dark Lord's favourite. For Arthas did not know this, but the Lich King was already planning how best to use his new toy.
But neither of them knew the outcome of the terrifying war that would soon rage across the sea, to the land of Kalimdor. No one would ever know the true outcome of a young prince's journey of vengeance; a journey that ended only by giving up what all man cherished.