The Lich King
The Lich King, the powerful king of the Undead Scourge, created by Kil'Jaeden to control it before the Burning Legion entered Azeroth.
The Lich King is born
To create such entity, Kil'Jaeden got the orc shaman, Ner'zhul, on his side. When Ner'zhul was defeated, in his anger, the Eredar warlock tore his body apart, but kept his soul, which he sealed, along with the Crown of the Lich King, in the Frozen Throne, at the top of the highest peak in Northrend.
From the Frozen Throne, Ner'zhul controlled the Scourge in Azeroth. In collaboration with his greatest operatives, including Kel'Thuzad, he slowly conquered the lands of Azeroth. It was then that he decided to take the matter of the paladin, Arthas Menethil, into his own hands.
Chosen by the Lich King
The Lich King created a powerful runic sword known as the Frostmourne. When young prince Arthas learned of its existence, he decided to set out to get it in order to use it to slay Mal'Ganis, destroying the Scourge, and thus saving Lordaeron. During his journey, in Northrend, he met an old mentor, Muradin Bronzebeard and set out with him to retrieve the blade. When Arthas learned that the blade was cursed, he disregarded it and everything else and picked it up, while at that very moment Muradin suffered from amnesia. Following those events, Arthas killed Mal'Ganis, then his own father. Leaving a sea of blood behind him and stopping at nothing, he worked for the Scourge as a Death Knight.
As Arthas was about to reach the highest peak of Northrend, he was blocked by Illidan Stormrage. As their fight went on, Arthas severely wounded him, then resumed his journey to the summit. Along the way, he was reminded of his, Uther's, Jaina's, and Medivh's words. Nevertheless, he continued up the mountain. He beheld the Frozen Throne, and within it, the Crown. At that time, Ner'Zhul, or rather his soul, was telling Arthas to return the blade, complete the circle, and free him from his prison. However, Arthas shattered the ice with a powerful blow, and the crown fell at his feet. He picked it up and placed it on his head. In that moment that Ner'Zhul uttered the words:
Now, we are One!
Arthas as the Lich King
Once Arthas had donned the crown, his soul was dominated by Ner'Zhul, who, from then on, freely controlled the fallen paladin's body and will, although Arthas himself, by the strength of his will, was able to hold back Ner'Zhul's destructive urges. Sometime after those events, the Alliance, and the Horde were jointly fighting the Scourge Army, when the leader of the Alliance army, Bolvar Fordragon, shouted:
Arthas! The blood of your father, of your people, demands justice! Come forth, coward, and answer for your crimes!After these words Arthas came out of the Citadel and replied:
You speak of justice? Of cowardice? I will show you the justice of the grave, and the true meaning of fear!Following the exchange, the leader of the Horde charged at him, and Arthas slew him. Upon witnessing this scene, the leader of the Alliance said:
You will pay for all the lives you have stolen, traitor!Unexpectedly, the Forsaken have appeared, and begun dropping bombs, thus ending the battle. Arthas has fled to his citadel.
The New Lich King
Following him, the heroes broke through to the Frozen Throne and begun fighting the ruler of the Scourge. Tirion Fordring became frozen at the start of the fight, and when it seemed that the Lich King was about to die, he defeated the heroes with a single blow. He wanted to resurrect them as Champions of the Scourge who would obey him. Then Tirion asked the holy powers for help. In answer to his plea, they gave him strength, shattered the ice that was imprisoning him, and broke the Frostmourne. The spirits trapped in the blade restrained Arthas, lifting him into the air, and his father resurrected the fallen heroes, who struck the final blow. Before dying, Arthas spoke with his father, and when he died, the King of Lordaeron told Tirion, that without a ruler, the Scourge would threaten Azeroth even more, so there must always be a Lich King to keep the undead in check. Tirion was willing to sacrifice himself to keep the undead under control, but then Bolvar Fordragon appeared and convinced him that it would be better if he became the new Lich King. Tirion crowned Bolvar, who, at that moment froze, then told him to say that the Lich King is dead, and Bolvar Fordragon perished with him.
The Lich King in Battle for Azeroth
Vol'jin, the late Warchief of the Horde, tried to find out who told him on his deathbed to name Sylvanas as his successor. He began to believe that it was not a loa who did this, but in fact something darker and more powerful. Vol'jin, Baine Bloodhoof, Talanji, and a Horde adventurer went to the Frozen Throne, to confront The Lich King about this. He told Vol'jin that he was not responsible and that Sylvanas was upsetting the balance of life and death. Furthermore, Lich King informed Vol'jin that he had been changed more than he had realized, and that he was neither undead nor damned, and did not belong to the Frozen Throne. But having said that, The Lich King sent the ghouls after them, forcing them to flee.
After the Fourth War ended, Bolvar created a new generation of death knights as his champions. He officially told those death knights that it was their burden and privilege to defend Azeroth, while being forced to endure the scorn of the ignorant. Then, he ordered them to speak with Darion Mograin and use his Death Gate to travel to the land of the living – to choose their destiny.
Serving as the Lich King, Bolvar Fordragon was visited by Sylvanas Windrunner – in the aftermath of the Fourth War, at the Frozen Throne in the Icecrown Citadel. Though they initially fought as equals, Sylvanas gained the upper hand. She eventually manages to subdue the Lich King and bind him with her magic. Deeming him unworthy of the crown he wore, she removed the Helm of Domination and destroyed it, shattering the veil between Azeroth and the Shadowlands. With the crown destroyed, the Lich King's position came to an end, leaving Bolvar with a fraction of the powers he once wielded, as well as the Scourge enraged and attacking Azeroth.