Plagued dragons refer to dragons that have been infused with the Plague of Undeath by the Cult of the Damned. The experiment began within Scholomance as another method of spreading the plague. The necromancers of Scholomance, working under Vectus, used black dragon eggs from the Burning Steppes as the basis for their experiments,[1] and hoped to unleash the plagued dragons on the world as a dragonflight.[2] However, the Argent Dawn caught wind of their activities and worked to destroy the abominable plagued dragons before they could be deployed.
Notably, both incarnations of plagued dragons are not listed as undead, meaning that they were meant to represent a sort of living draconic host; rather than the more commonplace undead dragons.
^Vectus says: We await a batch of black dragon eggs from the Burning Steppes. We believe that, through their study, we will advance our knowledge dramatically.