Once you have defeated both bosses the door opens up to allow you to follow on to find Arthurs. 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 where you can loot plenty of trophies so that you can make enough Dungeon Fighter gold. 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.
This is rebuff on the Lich King that slows him down significantly. It also causes a swirling storm around him that does frost damage to anyone that comes within 10 yards. Arthurs wields his deadly blade Frostmourne and will use it to unleash a furious attack hitting everyone for enough shadow damage to turn the mightiest tank to ash. He only uses this attack if you are too slow and allow him to reach you.