The Lich King was created by Kil'jaeden the deceiver, one of the lords of the demonic army The Burning Legion, as a means to conquer Azeroth. Kil'jaeden apprehended the fleeing orcish shaman Ner'zhul and enacted the blood pact he had previuosly sworn. The demon lord totally destroyed the orc's body and trapped his spirit along with a suit of armor and the runeblade Frostmourne in a block of ice atop the frozen throne at the bottom of a rift inside icecrown citadel in the continent of Northrend. The Lich King was given power over the undead. The dreadlords lead by Tichondrius were assigned to be the jailors of The Lich King.
Scourge of Lordaeron
Soon after that the invasion of the human kingdom of Lordaeron begins. The Lich King sends one of his necromencers Kel'thuzad to infect the granaries throughout Lordaeron. Arthas Menethil, the young crown prince of Lordaeron and a Knight of the silver hand is assigned by his father King Terenas Menethil to investigate the infections along with the young sorceress Jaina Proudmoore. As the party investigates they arrive upon a village preparing to defend against the undead inavasion. Seeing they are vastly outnumbered Arthas sends Jaina to his mentor Uther the lightbringer for reinforcements and prepares to defend the village with his small band of warriors. As the defenders are about to be overwhelmed, Uther and Jaina arrive with reinforcements to save the day. Next, the party marches towards the city of Stratholme. Upon arriving, they discover that the entire city has been infected by the dreadlord Mal'ganis and the population is turning into undeads. Arthas decides to cull the entire populace, however some of the knights including Uther and Jaina refuse to slaughter their countrymen. This leads to Arthas to disband and dismiss the Kinghts of the silver hand. Then, Arthas proceeds with the culling and fights back the undead. Arthas succeeds in sacking the entire populace but Mal'ganis escapes to the icy continent of Northrend. Arthas decides to follow the dreadlord to Northrend and takes the army with him. At Northrend, Arthas retrieves the powerful runeblade Frostmourne and uses its power to kill Mal'ganis and extract his vengeance. However it is revealed that the Frostmourne belonged to the Lich King and all the venets were being orchestrated by him all along. He wanted Arthas as his champion death knight.
Months later a fully corrupt Arthas returns to Lordaeron amidst a hero's welcome. However he murders his father in the throne room and starts the invasion of Lordaeron on behalf of the Scourge. Arthas then raises his dead steed "Invincible" and proceeds to kill his former comrades including his mentor Uther.
The Second Invasion
After the conquest of Lordaeron Arthas and his forces invade the elven kingdom of Quel'Thalas. Although he is met by heavy resistance from Ranger General Sylvannas Windrunner his forces gradually pushes towards the capital city of Silvermoon. Arthas finally defeats and kills Sylavannas but as a punishment for her insolence he raises her back as a banshee to do his bidding. He then defeats king Anasterian Sunstrider with minimal effort and his undead army rages Silvermoon to the ground. Arthas then resurrects the necromancer Kel'thuzad as Lich using the power of the Sunwell.
The pair then travels to Dalaran, seat of the mages. Although met with valiant resistance the scourge fights through the magical defenses and kills the archmage Antonidas. Lich uses the book of Medivh to summon the demon lord Archimonde to Azeroth and thus the spark the second demonic invasion of Azeroth.
The Frozen ThroneAfter the defeat of the demon lord Archimonde at Mount Hyjal by the combined forces of humans, orcs and night elves, Arthas returned to Lordaeron to reclaim his throne. He chased away the three Dreadlords ruling Lordaeron and crushed the human resistance along with Sylvannas and Lich. However during the battle Arthas has a painful seizure and feels his power diminishing and hears the Lich King calling out to him.
Arthas' powers dwindles and Sylvannas breaks free from his control and attends a meeting with the dreadlords to ambush and kill off Arthas. As Arthas fights off the dreadlords he is ambushed by Sylvannas and her banshees, however Kel'thuzad arrives just in time and chases away the banshees.
Soon after, Arthas received a mental cry from the Lich King who tells him to return to the frozen throne and defend it against the night elf Illidian who was been empowered by the demon lord Kil'jaeden to destroy it and his naga followers .Arthas gathered his army and headed for Northrend. After a desperate race against time Arthas faced Illidian at the foot of the Frozen Throne in a climatic battle. After he soundly defeated the fallen night elf Arthas ascends the Frozen Throne. Then he shattered the Lich King's icy prison and became one with the Lich King.
It is later revealed that Arthas deemed Ner'zhul's spirit to be too weak and suppressed it and became the dominant personality of The Lich King.