So far:
I reached the point where Arthas chases Jaina and the player. Jaina died twice before I finally managed to take down all the ice walls. Then I reached and empty room with no way to escape and everyone vanished, including Arthas. This prompted the game to kill me and disconnect me. When I logged in again, I was falling down the middle of nowhere and about to die of fatigue.
I used my hearthstone, talked to Loremaster Cho, and was sent back to the beginning of the halls of reflection, and the room was empty: no corpses, no mobs, nothing. I left the instance, talked to Loremaster Cho, and rejoined the story line.
Then I was sent again to the instance, but this time I was fighting Muradin, Jaina, and Uther as Arthas and going up the steps of ICC. At this point, I thought that I was past this part of the storyline. Nonetheless, I killed them around 40 times, stood in front of Terenas, fixed my gloves (as Arthas), and walked around aimlessly until I decided to leave the instance. I logged in again, was disconnected and sent back floating and falling a couple more times. When I log back in, I am fighting three ghosts aimless and can go nowhere.
I thought that this lore quest was about Arthas and not about being hacked and dumped.
If this is a WoW bug storyline, then it is working as intended.
I did this on my lvl 80 rogue, ilvl 662.