What is the story background of the expansion?

Players last visited Northrend in the Warcraft III expansion, when Arthas Menethil fused with the spirit of Ner'zhul to become the Lich King, one of the most powerful entities in the Warcraft universe. He now broods atop the Frozen Throne deep in Icecrown Citadel, clutching the rune blade Frostmourne and marshaling the undead armies of the Scourge. In Wrath of the Lich King, the greatest champions of the Alliance and the Horde will do battle with the Scourge amid Northrend's howling winds and fields of jagged ice, and ultimately face the Lich King himself.