This epic quest is part 5 of 5 the Deathtoll Access series of quests.
Steps
Return and speak to Nagafen in The Arbiter's Master instance of his lair. I can now enter Deathtoll and plunder its treasures.
Inside Deathtoll, in the Dragon's skull you will come upon a Barrier that cannot be breached (can be done solo - run around the monsters, try to reach the skull close to the entrance).
Go back to Nagafen. He will give you a scale and tell you to meet the armorer in Solusek's Eye (Keortor Talyse, same NPC as relic armorsmith. ( -69, -626, -59 ) Copy/waypoint -68.56, -626.30, -58.67. He will make a symbol that will allow you to breach the barrier.
It is possible to 'solo' to the flameshield in a 'suicide run'. Get see invis and see stealth of some sort (the tinkered goggles work nicely) and zone in and out of DT until you get a good instance -- one where the eyes are out of the way of the run from the left side to the skull. Then sprint to the skull, die, but get your update. Totem of the Owl and Totem of the Butterfly also gives the ability to see invis and see stealth. You could also use Vision Totem of the Cat if level 70+ that gives both abilities in one item.
