The Halls of Reflection is the third and final of the three new instances that make up the Frozen Halls in Icecrown Citadel.  The Halls were added in patch 3.3 which added all of the Icecrown Citadel content to the game.  You can only enter the Halls of Reflection once you have completed the other two instances: The Forge of Souls and The Pit of Saron.  This is so that you see and follow the whole storyline involved with finding the Lich King himself.

Once you have defeated both bosses the door opens up to allow you to follow on to find Arthas.  Once there you find that he is too strong and you must flee up the escape tunnel which forms the final fight in the instance.  This is another gauntlet type event where you must defeat several waves of undead as your faction champion clears the way up the tunnel allowing you to escape to safety.

Join Byron "Messiah" Mudry as he helps you complete the trilogy that explains the storyline behind the latest patch in the guide to the Halls of Reflection.