From wow.incgamers.com

Background

Prince Arthas Menethil, the new Lich King

Arthas was a human Paladin who in a quest to avenge the fallen of his kingdom took up the sword Frostmourne, which in turn stole his soul. By the dark powers of the sword he was turned into Undead servant of the Lich King, a Death Knight, and later merged with the Lich King to become the New Lich King, an enormously powerful entity that consists partly of the old Lich King, and partly of the man who used to be Arthas Menethil. Arthas is the current leader of the Scourge, and the Cult of the Damned hails to him as well.

Introduction

Arthas the Lich King resides in Northrend, the continent to the North in the world. Characters have to be level 68+ to be able to visit this area. This continent is currently unavailable to players until the expansion: Wrath of the Lich King is released on November 13th 2008.