Quest: Black Wyvern Down (Part 9)
Finally, we are ready to cleanse the large poison brewing pit! Curate Sabha's scouts have seen increased Ksaravi activity around the large pit, we think they're getting ready to distribute the Feverbane! Oh no! Quickly, travel to the largest poison brewing pit. The presence of the Coronarian Censer in your inventory will force the essence of the poison pit to manifest so that you can fight and destroy it. This will cleanse the pit and destroy the Feverbane once and for all!

So get on your wyvern and fly to the large pit once again. When you land in the pit, an Essence of Feverbane (level 17 4dot) will spawn somewhere in the pit. Find it and destroy it (Watch out for the other mobs in the pit. They will add if you are not careful!).
NOTE: If the Essence of Feverbane despawns before you attack it, just fly out of the pit, and land back in it, to spawn the Essence again.
After the Essence of Feverbane is destroyed, head to a safe spot and right click the Coronarian Censer once again to complete the quest, and speak to Curate Sabha.