Place the luclinite chunks around the portal. Look for the blue sparkles.
Defeat the Unstable Avatar.
Strategy: The mob is epic x2. Cast the spell given to you, Celestial Avatar, as soon as he is available. He lasts for 20s only and allows you to hit the mob more strongly. There is also a powerful health regen tied to it. When the spell is not up, you'll have to rely heavily on heals. Around 10% of the Avatar life, Unholy powers will tear through you but form will endure it. Barely. Save a major healing spell for that moment. Save the Celestial Avatar buff for that moment, if it's not active, the spell will probably kill you. You will be knocked back through the air but the mob seems to be rooted for some seconds, allowing you to survive.