Go to the Avalanche in Sholazar Basin and use the Omega Rune to deploy Etymidian. Use him to slay 200 Scourge Minions, Bythius the Flesh-Shaper, Urgreth of the Thousand Tombs and Hailscorn. Speak to the Avatar of Freya in Sholazar Basin when you've completed this task.
200x Scourge Minions destroyed
It is time, [player]. The Etymidian is one of the most destructive forces ever created by the titans and I am loathe to unleash him in my lands.
There is little choice, however. Our enemies have forced my hand and the fury of the titans will cleanse all.