my theory on what will happen at the end of IC citadel is this:
At say 3% or 1% health, all us players drop out of the fight and Tirion closes in on arthas to finish him off. As they battle, Tirion knocks frostmourne out of Arthas's hand and catches it himself, thus frostmourne taking over Tirion's soul and making him the new Lich King. Tirion they delivers the final blow to arthas, killing him.
As soon as arthas dies, it creates a massive blast of holy energy cracking the foundations of the citadel, and it is about to collapse. With a list bit of humanity left, Tirion tells the players to leave the citadel before its too late.
Then once the players get out, there's an epic use of phasing as the entire citadel collapses, crushing everybody inside, including Tirion.
Right after the destruction however, there is another cutscene that plays. It shows your character walking towards the rubble and as they look through it, they find the hilt of frostmourne,the blade missing. As you pick it up, a thought in the chat box comes up: You must take this to be reforged as a weapon of your victory. (i.e. Once you take it to a certain NPC , they will reforge it to make Shadowmourne)