Slay Elite Aviator Raschaud Tuthosols (lvl 75^^^ , but hits harder) just outside of Riliss ( -104, -26, -152 ) /waypoint -104, -26, -152 in Fens of Nathsar.
Find a way to spread the fermenting agent across Riliss. After Raschaud's death, get on his Sokolar named Burkmaiir, and rain destruction from the sky. (Note: there's nothing you need to actually do while riding the Sokolar, just enjoy the show until the ride ends.)
Find and slay Invigilator Warwyn Al'Xi (lvl 75^^^) located ( 1300, 200, 1087 ) /waypoint 1300, 200, 1087. Then use his Sokokar and poison the Sarnaks. (Note: again, there's nothing you need to actually do while riding the Sokolar, just enjoy the show until the ride ends.)
Head to the Field of Bone. Need to be at Jaled Dar's Remains . The discovery location is ( -1437, -371, -1488 ) /waypoint -1437, -371, -1488, however the spot you need to be to summon Hrath again is in the center of the skeletal dragon's rib cage at ( -1545, -369, -1524 ) /waypoint -1545, -369, -1524
Use the item Scroll: Call of the Plaguebringer to Summon Hrath. Speak to him to start him summoning Bertoxxulous. Harath will be attacked by three waves, the first two are 5 grouped level 75 followed by a named (High Priestess Jhanda), you need to protect Hrath while he summons Bertoxxulous.
Once the Priestess is dead, wait a moment and The Avatar of Plagues will spawn. Hail him, go through the text, and the quest will complete.