Purchase a Husk: Primal Evolution from Quartermaster Queb for 800 Planarite. This requires Decorated status with Order of Life Serene. Craft the Lure: Primal Evolution from the Husk and five Malformed Souls which can be found in tier 1 expert dungeons.
Travel to Stillmoor and find any none raid tears. Use the Husk to start the Expert Rift: Primal Evolution. Complete the rift, all the way through the bonus stages to face off again Duke Ellisar. Use the Diona's Book of Aqueous Rituals on him to obtain the final step of the quest, to kill him.
Slay him and rejoice in your hard earned rewards. You will also complete Valley of Darkness and gain the title Redeemer.