Das Ritual

Yonas Alazar sends us to seek out the ritual side within Amrine Temple.

Go up the stairs on the north side of the building.
On top will wait a named enemie, Kar-El the Gate Keeper (kill not requiered).
Behind him you see the Soulwarden Seal, pass through it.

Inside the game wants us to defeat the Heartforge Guardian.
It will be imune to your damange until you ignited the four heartgem flames around him.
As soon as his HP hit 0, he will give up and tells you to complete the ritual.

After you are done, jump down and head back to Yonas Alazar.

At this time on the PTR, this quest requires defeating the Heartforge Guardian. It sits at the top of Noblereach inside a portal, and not in the expedition as the quest text suggests. - https://ptr.nwdb.info/db/creature/ancientguardian_mage_msq_named_02_encounter_boss