One of the most amazing things about World of Warcraft is how well its backstory ties in with Blizzard's Warcraft RTS games. The level of detail in its stories and characters, and they way they all interact, is astonishing.
So a working knowledge of Warcraft III may not give absolute meaning to your endless monster killing, but at least makes you feel part of a glorious whole.
The Lich King was the main human character of Warcraft III - the paladin Prince Arthas Menethil. Arthas's downfall began in the fifth mission of Warcraft III in the city of Stratholme, where the entire population had consumed grain poisoned by the Undead Scourge, dooming them to a pallid complexion for evermore.
Stratholme is a WOW dungeon these days, somewhat revived since Arthas culled the entire town before consequently hightailing after the nasty Dreadlord responsible, Mal'Ganis, to Northrend. Which is where the new and exciting northern continent of Azeroth first got a look in.
Arriving on the shores of Northrend, Arthas shared pleasantries with some Dwarves, who let slip about a powerful runeblade called Frostmourne. Before they could search for it, though, a call came through from the human city of Lordaeron (currently sitting in ruins atop the Undead capital of Undercity) telling them to come home.
The troops attempted to return, only to find a crazed Arthas had decided to set fire to their ships (blaming the mercenaries he had hired and murdering them) stranding his men. This was the first sign that he was going mental, and another sign that there'd be a ton more top-down RTS warfare before the day was out.
Arthas later found the blade in Drak'Tharon Keep and, ignoring the large sign saying "Warning, cursed blade, will destroy your soul," took it, got a curse and lost his soul. It soon turns out that the Dreadlord Mal'Ganis had been sent by the Burning Legion (the Burning Crusade demons who want Azeroth destroyed) to corrupt Arthas into becoming an evil Lich King in just that fashion. So Arthas goes on to stab him in protest.
GO NORTH YOUNG MAN
The story then has a few months pass, before Arthas returns home with a somewhat deeper voice and mad eyes. He kills his father, the human king, and runs away, gaining followers in the form of The Cult of the Damned.
He leads the mindless Undead Scourge around Azeroth and ends up creating the sex-hungry mana-spilling Blood Elves (later to become one of The Burning Crusade's playable factions) by wrecking that Sunwell they bleat on about - in fact using it to bring a necromancer mate back to life and half-destroying Silvermoon City in the process.
Arthas then gets passed a message from his newly reanimated necromancer buddy that the Burning Legion's master, Archimonde can be summoned through a spell book residing in peaceful environs of Dalaran. Cue summoning of big nasty demon, cue destruction of the city-state of Dalaran (at least until it returns as a flying city in Lich King).
At this point Warcraft III expansion pack The Frozen Throne enters the catalogue of lore. Due to mysteriously fading Lich-puppet powers, a splinter group broke away from the Undead Scourge and called themselves the Forsaken (who are now a sentient, playable Undead faction in WOW). Concerned, Arthas heads north, where the Frozen Throne of Northrend is under attack by the Blood Elves.
Arriving in Northrend, Arthas battles away: annoying Dwarves and the claret-eyed Elven forces of Illidan Stormrage (who ultimately becomes the corrupted boss of the first WOW expansion pack The Burning Crusade).
The last you see of Arthas is him walking towards the Frozen Throne, where he puts on the Lich King's helm and fuses with the Lord of the Undead, creating a physical form for the long-ethereal creature. So, basically you should get levelling, because he's not going to kill himself, is he?
Coming soon: Part two - Blizzard spills hot beans on the new World of Warcraft expansion.