"Defeat any Plague of the Well while Ascendant"
Pretty self explanatory, however I've defeated about six of them (yes, while ascendant) and have not been able to complete the quest..
Is it incredibly specified, where I have to land the final blow? If that's the case, players are better off trying to solo tier one, as it's almost impossible to land a final blow in a 3-6 player active Well event.
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Plague of the Wells only appear in tier 3&4 (I believe) You may be mistaking the Servant and Heralds of the Well for the Plague (I did that myself for embarrassingly way too long....)