Arthas and Illidan contest the Icecrown obelisks

Full History

Arthas and Illidan fought for control of the magical obelisks surrounding Icecrown, whose power opened the way to the Frozen Throne. The struggle brought two armies to the same frozen fortress with opposed purposes. Illidan intended to destroy the Lich King's prison; Arthas sought to protect it and reach his master.

Illidan approached with naga and blood elf allies after Kil'jaeden renewed his demand that the Lich King be eliminated. Arthas arrived from Lordaeron with Anub'arak's assistance and the urgency of a death knight whose own powers had faltered. Securing the obelisks was necessary before either commander could reach the prison at the summit. The fighting therefore made the fortress itself part of the battle rather than merely the place where the final duel would occur.

Arthas ultimately prevailed and defeated Illidan before climbing to the Frozen Throne. The contest of the obelisks is the last major military stage of that ascent. It explains why the fate of Ner'zhul was determined not by distant magic alone but by armies struggling to control the approaches to a specific, heavily defended place.