This quest is not available in game.
Destroy the Ancient Moonstone Seal at the ancient flame in Ameth'Aran, then return to Asterion in Bashal'Aran.
It was the craft of one of the most powerful of the Highborne that created the seal that formed my prison. In Ameth'Aran, the ruins to the south that are twin to these, persists even today an ancient flame, blue in color. In this flame this seal could be destroyed.
Be wary in the ruins, <name>...