After stumbling through an entire content and possibly doing more harm than good, you've finally arrived in Icecrown, the doorstep to Icecrown Citadel, which is where Arthas resides.
Of course, getting to it isn't as simple as just walking up to the front door. Icecrown Citadel lies in the south, near the center of the zone, and you are only able to enter from the southeastern corner, after the disaster at the Wrathgate in Dragonblight. Your entrance to ICC is blocked by a large mountain, and the vrykul city of Ymirheim, so you have to take the long way around. And because there wasn't enough in your way, giant gates are located every so often to block your progress.
The first foothold in Icecrown is the Argent Vanguard, where Tirion Fordring Argent Crusade have managed to create a base of operations. You help them ward off nerubians and waves of undead forces, and even establish a new base, Crusaders Pinnacle, in the nearby mountains.
After your success, you report to your faction's next base. However, due to the giant gates, and fields of Scourge forces, they aren't on land...they are flying high about Icecrown in airships. The Alliance's Skybreaker...
and the Horde's Orgrims Hammer.
From high in the air you are given missions all around the zone that have you striking at all of Arthas' operations in order to render him as defenseless as possible once you finally attack the Citadel. You free several of his high-level minions from their servitude and take over their base, the Shadow Vault, as a staging ground for the friendly death knights of the Order of the Ebon Blade.
You are able to dismantle the Fleshwerks, a ghastly factory where Arthas is able to churn out abominations and flesh giants...
Stop the production of a floating necropolis....
Cut off Arthas' support from the giant vrykul by shutting down their major city and bases located throughout the zone...
And help your ground forces by thinning out the Scourge forces between the gates.
Eventually, you are given a task to set off a bomb underneath Icecrown Citadel. Something goes wrong, however, and you are thrown into a deep pit, and when you come to, well...things start to get a little weird.
In your attempt to find a way out of the pit, you come across an object, and immediately upon touching it, you recoil in pain, and immediately hear a whisper telling you that you shouldn't have touched it. It turns out that the whisper came from a boy nearby.
The boy, named Matthias Lehner, informs you that "he" knows that you touched the object, and is coming to get you. He helps you portal back to the surface, where you are reunited with your forces, who thought you were dead. This doesn't stop you from sending you on your next mission, however, and you are given a siege engine and told to kill as much Scourge as you can. The little boy appears again, however, and tells you that no matter how many you kill, you can never win. He then shows you how Arthas killed his own forces and raised them up as Scourge and how he was able to raise the frostwyrm Sindragosa from death. His message is that no matter how many soldiers you send to kill Arthas, they only increase his forces. Finally, he meets you in the Court of Bones, just outside Icecrown Citadel, to show you Arthas' encounter with Illidan on his way to the Frozen Throne.
Lehner tells you that despite winning the battle, Arthas realized that his heart was making him weak, and upon becoming the Lich King, removed his heart and cast it deep into the heart of Icecrown where it stayed...until you touched it. Now remembering its presence, Arthas is seeking to reclaim the heart and keep it safe. You inform Tirion Fordring of this, and he comes up with a plan to steal the heart while it is being transferred from its resting place to a place in Icecrown called the Cathedral of Darkness.
You and Tirion disguise yourselves and infiltrate the cathedral. As the procession enters with the heart, Arthas appears, able to see through your disguises, and confronts Tirion. Tirion determines that any chance that Arthas could be redeemed is gone, and in one swift motion, destroys the heart, stunning himself and dramatically weakening Arthas.
Armed with the realization that brute force is not the answer to stopping Arthas, Tirion creates a large tournament arena in the northern part of Icecrown, and begins the Argent Tournament. Its purpose is to determine which of the Horde and Alliance heroes are most worthy to face Arthas in Icecrown Citadel, and who will give the Argent Crusade the greatest chance at victory.