Lich King

"Young heroes... I was once like you. You have come to this place seeking to bring judgement upon the damned. You will venture deep into forgotten lands. You will see wonders beyond imagining. But be warned. The land itself will rise up against you. Long forgotten terrors will smother your courage. Sacrifice everything as the final darkness falls... in the end, all that awaits you is death. Only then will you understand - you've been following in my footsteps all along. So come then, you heroes! Come in all your power and glory! For in the final hour, all must serve the one... true... king."

- The Lich King in the trailer for Wrath of the Lich King.

The Lich King is a powerful entity of pure evil in the Warcraft universe who is the supreme leader of the undead army known as the Scourge. He is not so much a character as an evil force created by the demon lord Kil'jaeden, originally as a tool to control the Scourge, which Kil'Jaeden originally created to further the ends of the Burning Legion but that later grew completely out of control and became an equal or greater threat than the Legion in their own right. He serves as the titular main antagonist of World of Warcraft: Wrath of the Lich King.

History
The Lich King's only physical forms are a cursed suit of armor and a sword called Frostmourne, both of which are enchanted with the element of ice, and requires a mortal host to do its bidding, who becomes the Lich King when they put on the Helm of Domination. The Lich King is completely immortal and eternal, and when one host is killed a new one has to take their place, or else the Scourge will run completely rampant without a master and destroy all life on Azeroth. The Lich King rules from the “Frozen Throne” atop the Scourge's fortress of Icecrown Citadel in the frozen continent of Northrend, where he spends most of the time hibernating/doing nothing.

The original host used to create the Lich King was an Orc Shaman named Ner'zhul, but the individual best known for filling the role was Arthas, who became corrupted by the Scourge and coerced by the Lich King's spirit to come to the Frozen Throne and put on the Helm of Domination, turning him into the new Lich King. The form and personality of Arthas would become dominant over that of Ner'zhul.

After that, the Lich King went into hibernation atop his throne again for several years, before finally returning to wreak havoc on Azeroth with a newly revitalized Scourge army in Wrath of the Lich King. At the start of the expansion, the Lich King is at the necropolis of Archerus: The Ebon Hold in the Eastern Plaguelands, training a group of death knights who eventually break away from the Scourge and join the side of good, being the Death Knights controlled by player characters. During the campaign against the Scourge in Northrend, the Alliance and Horde simultaneously launch a massive attack on the Wrathgate in Dragonblight leading into Icecrown in an attempt to assault the citadel, where after defeating all the monsters outside the Alliance general Bolvar Fordragon called out for Arthas to show himself and answer for his crimes, which he did, but the confrontation was interrupted and turned to disaster when a group of rogue Forsaken led by Grand Apothecary Putress arrived and launched barrels of poison gas on everyone, killing all the Alliance and Horde soldiers (including Bolvar and the Horde hero Dranosh Saurfang) and forcing the Lich King to retreat further into Icecrown.

Eventually, the heroes finally secured a position to launch a final attack on the Lich King in Icecrown Citadel. The first battle took place in the Halls of Reflection, where Frostmourne is kept, in which the Lich King appeared and tried to stop them from escaping by summoning groups of Scourge and ice walls, but the adventurers survived the encounter with the help of Sylvanas Windrunner/Jaina Proudmoore (depending on the player's faction of choice) and the convenient arrival of an airship to save them. A much larger and stronger group of heroes later fought the Lich King for the final time on his Frozen Throne on top of the Citadel, where they were aided by Highlord Tirion Fordring of the Argent Crusade.

After a considerable period of fighting, the Lich King finally unleashed the full power of Frostmourne, killing everyone. Luckily, Tirion Fordring jumps in at the last minute and saves the day by breaking Frostmourne with his holy sword of the Ashbringer. This frees all the souls of the Lich King's victims that were trapped inside, including Arthas' father Terenas Menethil, who used the power of their spirits to revive the group and overwhelm the Lich King, allowing them to finish him off. The Helm of Domination fell from Arthas' head, allowing him to finally die. However, there was now the matter of finding a new host to sate the spirit of the Lich King and prevent the Scourge from destroying the world; this burden was taken up by Bolvar Fordragon, now an undead being who had been imprisoned by the Lich King, who donned the helmet and encased himself in ice upon the Frozen Throne, pacifying the Scourge.