So apparently Ner'zhul and Arthas didn't have a single original bone in their bodies and just copypasted everything they saw in the Shadowlands into Azeroth.
Plagues, Liches, abominations, death knights etc from Maldraxxus.
Val'kyr from Bastion.
San'Layn and gargoyles from Revendreth.
Hell even the architecture is taken from Maldraxxus and the Maw. This makes the scourge feel rather cheap, I don't like it.