The entity known as the Lich King is actually a fusion of the orc warlock Ner'zhul, and the human paladin turned death knight, Arthas Menethil. Ner'zhul, a servant of the powerful demon Kil'jaeden, was a major leader of the Orcish Horde during the orc invasion of the world of Azeroth. Near the end of the Second War between the Horde and the Alliance of Azeroth, Ner'zhul set about building portals to other worlds, madly hoping to lead a mass invasion through these portals. His plans were interrupted by a reprisal by the Alliance, disrupting the portals and causing them to rip apart the orc homeworld of Draenor. Ner'zhul narrowly escaped through one of his portals, ending up on the Legion's homeworld.
Here, Kil'jaeden punished Ner'zhul severely for his failure, destroying his physical body and torturing his soul. Eventually, Kil'jaeden decided to give Ner'zhul another chance. Binding Ner'zhul's soul to a mystical suit of armor, which he imprisoned in a block of ice, Kil'jaeden dispatched the warlock to Azeroth, where he landed in the frozen land of Northrend. Here, Ner'zhul discovered he possessed untold mental powers, as well as the ability to use a magic plague to rise creatures from the dead as his personal army. Thus was the first Lich King born, beginning his quest to replace all life on Azeroth with his undead minions.
The champion Ner'zhul chose to lead his army, as well as aid the Lich King in escaping his frozen prison, came in the form of Prince Arthas, son of King Terenas of Lordaeron. Serving as a paladin of the Silver Hand, Arthas at first fought against the Undead Scourge, but soon became obsessed with driving them out and protecting his people at any cost. In Northrend, Arthas discovered the runeblade Frostmourne, which he believed would aid him in destroying the undead.
In fact, the runeblade had been planted by the Lich King, who sought to corrupt and control Arthas. He succeeded, and Arthas murdered his own father in cold blood upon his return. Arthas became the first death knight of the Scourge, slaying many and destroying several powerful nations. But his true destiny was yet to unfold. Advised by his close "friend", the lich Kel'thuzad, Arthas succeeded in overthrowing the Lich King's demonic supervisors, the dreadlords, and undoing the Burning Legion's invasion of Azeroth, which Kil'jaeden had planned on using the Scourge for.
Ultimately, Arthas journeyed back to Northrend, defeating Illidan Stormrage, who was staging an attempt at destroying the Lich King. Arthas shattered the Frozen Throne and put on the Lich King's armor, merging with Ner'zhul's spirit to create an entity with Arthas' form, but the combined minds of Ner'zhul and Arthas. More powerful than ever, the Lich King now wages war with the intention of annihilating the living.
Battle vs. Mephisto (Marvel) (by CuchulainSetanta)Edit
In Mephisto's palace in the Stygian Deeps, Mephisto is angered to learn that an entity known as the Lich King is taking souls that Mephisto could use for his own. Transporting himself to Icecrown, Mephisto arrives right in front of the Frozen Throne. Seeing this threat to his rule, the Lich King rises, unveiling Frostmourne and summoning a legion of undead to attack the intruder.
With a simple wave, Mephisto blasts a wave of hellfire at the Scourge forces, incinerating them. The Lich King is surprised, but steps down to meet the hell-lord in combat. He thrusts forward with Frostmourne, but Mephisto evades by teleporting behind him. Gathering another wave of hellfire, Mephisto blasts the Lich King, knocking him to his feet.
Mephisto approaches the fallen King, but is stopped in his tracks when the Lord of the Dead summons a blizzard, buffeting Mephisto with hurricane-force winds. Shouting in fury, Mephisto blasts the blizzard back with a burst of hellfire, then leaps for the King. Readying himself, the Lich King catches Mephisto on his blade, causing the demon to explode in a shower of hellfire.
The Lich King laughs, walking back to his Throne, only to be knocked to his feet when the Citadel shakes violently. Getting back up, the King walks to the edge, looks down, and sees Mephisto down below, sucking Icecrown Citadel into a portal down to Hell; the Mephisto who had been fighting this entire time was just an illusion. Another tremor sends the Lich King plummeting down into the inferno, with Mephisto catching Frostmourne as it hurtles past. Mephisto absorbs the countless souls within the blade, cackles, and then disappears to torture his new prisoner.
Mephisto won because his power levels far exceeded the Lich King his shapeshifting abilities allowed him to get an edge on his foe, and his Hellfire was naturally stronger than the Lich King's Ice and Snow. It was also believed that, being the Marvel Satan, that a God-level being was required to best him in combat, and the Lich King simply wasn't at that level of combat prowess.