Bring 6 Mathystra Relics to Onu at the Grove of the Ancients.
The parchment you found at the Master's Glaive is a page from an ancient text...written by elves! It is heavily cursed and tainted by the corruption of the Twilight's Hammer, but it seems the cult is using ancient elven magic to bring their old masters back to our world.
Travel to the ruins of Mathystra to the northeast, and search for ancient relics among the grass and broken stones. Through their study, we may divine how the Twilight's Hammer intends to exploit the magic of the elves.