Destroy all of the infected corpses in Ferndúr's chamber before defeating the Gaunt-lord himself.
This quest will be cancelled if you leave the Lost Temple before completing it.
Ferndúr the Virulent remains scheming deep within the Rhudauran temple.
Ferndúr the Virulent has nearly completed his deadly experiments beneath the lost Rhudauran temple.
The plagues he has concocted are too deadly to be allowed to fester. You must destroy all the corpses within his chamber before the Gaunt-lord himself is defeated in order to ensure that every trace of them is obliterated.