From WoWWiki

The War of the Spider was a conflict between the fledgling undead army of the Lich King and the nerubian kingdom of Azjol-Nerub in Northrend.

While the nerubians were immune to the Lich King's plague of undeath and powers of mental domination, their subterranean kingdom eventually fell to the waves of undead and the cunning Nathrezim. Following the conflict, the Lich King incorporated the nerubian crypt fiends and Crypt Lords into his ranks, and adopted the nerubian architectural style as his own.

For the in-game book and section of the History of Warcraft providing an official account of this event, see War of the Spider (History of Warcraft).