The Pantheon of Death wasn’t created by the First Ones but summoned into Robot Host Bodies programmed to follow the First Ones’ Purpose!
Furthermore Il’gynoth states that the Father of Sleep(Mueh’zala) will have his Feast…
Yogg-Saron’s Magic looks suspiciously similar to one of the Magics used by Mueh’zala: his Necromantic Magics.
G’huun’s Blood Magic also resembles Mueh’zala’s Blood Magic.
The Void Lords didn’t simply send mere Servants but summoned the Gods of Chaos, Madness and Death themselves into their agents simply to increase their corruptive power!
Zovaal also mentions Death coming for the Soul of our World and was about to preform a ritual on Azeroth…
He was about to summon Mueh’zala/Yogg-Saron/G’huun into the World Soul!
The next plan for the Old God of Death(as opposed to that upstart the Primus) is to use Vol’jin to access Azeroth through the Emerald Dream upon his resurrection then enter Dragon Isles and through the Bronze Dragonflight’s territory gain entry to AU Draenor and obtain the World Soul.
“The Light has struck a bargain with the enemy of all.” - N’Zoth
The God of Death, Shadow and Blood is returning in a Planet Sized form(probably summoning his Son the God of Sleep too) and the God of the Deep is terrified even as we purge his Voidspawned Avatar in Ny’alotha!