The name of Icecrown became infamous when Kil'jaeden hurled the Lich King back into the mortal world. Ner'zhul's prison slammed into the glacier, creating the Frozen Throne. Here the Lich King remained until Arthas freed him and the two beings merged in Arthas' body. The merging created such a large explosion that a large portion of the glacier was blown away, leaving a gigantic throne in its place, upon which sits the Lich King. From here he rules the Scourge with an iron fist.
From the name one would assume Icecrown Citadel was an enormous fortress planted on the glacier, with thick walls, sturdy guard towers, and massive front gates. Not quite. Most of the citadel lurks below the surface. The glacier is split, creating a deep fissure. Scourge forces lurk down there, and they have carved levels into either side of that massive split and created bridges, walkways and even solid floors across the gap. Not surprisingly, Icecrown Citadel is a cold, unforgiving place. It has no decorations, no furs or rugs, no fires, nothing to provide comfort or warmth. The Scourge creatures move silently from one problem to another, focused on their tasks. All manner of undead dwell here, including zombies, wraiths and beings composed of cold energy. At the bottom of the rift is the Frozen Throne, the heart of the Scourge and the Lich King’s personal seat. This throne is the remains of the block of ice that first brought Ner’zhul to Azeroth, the same block that held him prisoner until Arthas freed the ancient orc shaman and merged with him. Arthas sits on the Frozen Throne. ( )
This description may seem confusing, as images of the Citadel obviously depict a spire with the Frozen Throne at the top. This is more understandable when the size of the rift created by the impact with the Glacier is realized. The rift is so large the entire spire is below average ground level. The entire spire counts as the Frozen Throne, and Icecrown Citadel is the fortified rift as a whole.
Wrath of the Lich King
Icecrown Citadel will likely be opened in World of Warcraft: Wrath of the Lich King, featuring the Lich King as the final boss, though it is still unknown if he is going to be killable or not.