1. Mal'Ganis' Corruption: Arthas' journey begins when the demon lord Mal'Ganis corrupts the Helm of Domination, an artifact of immense power. Arthas seeks the cursed artifact to lift a plague from his kingdom of Lordaeron, but upon finding it, he falls victim to Mal'Ganis' manipulations.
2. Loss of Humanity: Arthas' desperate pursuit of the Helm leads him down a dark path. He sacrifices his humanity and sense of morality to acquire the artifact, believing that it is the only way to save his kingdom.
3. Merging with the Helm: Upon claiming the Helm of Domination, Arthas' soul becomes permanently fused with the artifact. This fusion grants him immense power, but at the cost of his free will and individuality. He becomes a puppet controlled by the helm's malevolent influence.
4. Ner'zhul's Spirit: Trapped within the Helm of Domination is the spirit of Ner'zhul, the former lich king and leader of the Scourge, an undead army raised by the Burning Legion. Ner'zhul's spirit merges with Arthas' essence, further corrupting him and amplifying his power.
5. Control of the Scourge: With the combination of the Helm of Domination and the spirit of Ner'zhul within him, Arthas gains control of the Scourge. He becomes the new lich king, leading this vast undead army with the goal of purging Azeroth of life and ushering in an era of eternal undeath.
6. Death Knight Creation: Arthas also possesses the power to create new death knights, individuals bound to his will through a cursed runeblade known as Frostmourne. These death knights serve as his loyal enforcers and champions in his quest for supremacy.
As the King of the Undead, Arthas seeks to conquer all of Azeroth, eliminate the living, and reshape the world according to his warped vision of order and control. His transformation marks a pivotal moment in the Warcraft saga and sets the stage for epic conflicts between the living and the undead forces in subsequent Warcraft games and expansions.