It was a very catchy tune.

Arthas frowned from atop his throne. He had important things to be concerned with, but he couldn't seem to get the blasted song out of his head! It had been stuck in his mind for days without reprieve. To make matters worse, the Lich King didn't know all of the lines - only the tune, stuck on never-ending repeat in his head.

All of the abominations had learned the song and were humming it now. Even a few of the geists had picked up the habit. Since the lower level servants in the Citadel were constantly being rotated out with those from the front line, the song had spread throughout all of Icecrown. The higher ranking undead, at first annoyed by the song, now reported with malevolent glee that it was terrifying the Argent Crusade. Arthas' favorite report was of an abomination who had ploughed through a squad of paladins, all while humming vigorously. Let the survivors report that back to Fordring, he thought with malicious amusement. He was sure the older man wouldn't know what to think.

The panic the song was causing amongst the Lich King's enemies was well worth the minor annoyance of having it stuck in his head. Still, Arthas thought that the abominations had it easy. They were free to sing the song to their heart(s)'s content, to get it out of their little minds. If Arthas were to start singing, or even quietly humming, he was sure there would be consequences. His council would think he had lost his mind - and Arthas could not afford to lose their support, not with his forces threatened on all fronts.

...

After a week without relief, the Lich King was almost to the point of not caring what the council though. He had to get the song out of his head! Without knowing the words, his mind had begun to fill in the blanks with his own lines. It was driving the Lich King mad.

Still, he was determined to be subtle. At the end of a meeting, the Lich King told his advisors that he intended to survey his forces. He turned down all offers of accompaniment, and sent everyone off on tasks to make sure they were kept occupied during his absence. It wouldn't do for one of them to decide to check in on their master in their spare time...

That task taken care of, Arthas took the transporter to his frozen throne. After a moment's consideration, he let out a sharp, summoning whistle. Though he could easily open portals of frost to travel across Northrend, Arthas wanted to conserve his power. The forces of the living were proving to be more resilient than he had thought, and the Lich King was determined to use all of his power to stop them.

Besides, he thought with a rare smile, this way he got to see an old friend.

A hollow whinny alerted him to Invincible's approach, and Arthas turned to greet his oldest and most loyal friend. Glowing eyes looked at him with adorations as he stroked his mount's nose. The undead horse seemed unbothered by the frost that clung to his master; when Arthas mounted him, the horse made no sound of complaint at the weight of the Lich King's armor.

The smile remained on Arthas' face as they took off. It was very rare for him to ride Invincible now; he had missed it.

True to his word, Arthas surveyed his forces from the air. The grin he wore disappeared as he did so. Matters were more dire than he had thought: the factions that fought against him had gained ground.

Troubled by this, Arthas turned Invincible mid-air to head back Icecrown Citadel - but the song suddenly resurfaced in his mind with a vengeance. Reminded of the original purpose of his trip, Arthas let out a frosty curse and turned Invincible once more, this time towards the Storm Peaks.

It took a while to find a spot that fit his needs, but Arthas eventually let Invincible land on a mountain top. It was some distance from the grounds of the Argent Tournament, on the border of Icecrown and the Storm Peaks. The distance was good, as was the storm that raged upon the mountain - it ought to swallow his voice and keep anyone from hearing what the Lich King was about to do.

With a deep breath Arthas dismounted, patting Invincible as he did so. He drew Frostmourne and plunged it into the snow, keeping one hand upon its hilt. The empty spot in his chest leapt at what he was about to do. Unsure of the original words, Arthas gave voice to those his mind had assigned to the tune:

My eyes glow blue in their sockets tonight

So inhuman - can't you see?

A kingdom of death and mayhem

And I've made myself its King

Frostmourne is howling with the souls locked up inside

Invincible, upon him I'll ride