It is clear to me now that there must be a Zereth for each force of the cosmos, and within them, an inner sanctum akin to the Sepulcher.
And if that is true, then these sanctums must be connected on some fundamental level; a connection that the Jailer sought to exploit.
What he began in the Sepulcher was meant to cascade outward from one Zereth to the next, until all were dominated by his power. The heart of the Shadowlands acting as a doorway to the heart of each cosmic force in turn, one after another bound to his will.
But if his scheme was thwarted, then why is my mind so ill at ease?
Because I have seen how fragile the pattern is. How delicate the scales that keep the six forces in balance.
And if the Jailer, in his act of malevolence, left behind but the most imperceptible of cracks in that pattern, then I fear what is tiny now will only grow, until the balance itself is prone to be shattered by another force applying relentless pressure.
Could it be possible that the Old Gods wanted to be killed, to be sent back to the Realm of the Void so that they could enter into the Void’s Zereth and break through to other cosmic realms?
Is this the gateway for the Void Lords?
Would N’zoth being killed by the Heart of Azeroth carry some of that essence back with him to the void, to interact with a relic of the First Ones?
I’m gonna say it’s unlikely. The Old Gods it turned out were more or less sentient bio-weapons of the Void Lords, fired off in any direction, hoping to hit a target with a world soul for corruption. Our old gods happened to hit Azeroth and followed their programming as “smart weapons”; dig into Azeroth and corrupt the soul. Even N’zoth “died” (allegedly) working towards that goal.
Additionally, Zovaal could only gain access to Zereth Mortis because he had his infinity gems all collected. Unless the old gods happened to find all of the void’s infinity gems which happened to all be found on Azeroth and they happened to go with them post-death, then they wouldn’t have a means of copying Zovaal’s plans. And that’s almost too many coincidences even for a Blizzard narrative.
That’s just my take on it, but the easy counter is “Blizzard literally just made two expansions rehashing prior plots beat-for-beat, they could easy rehash this one with the old gods” and I cannot argue against that.
They got in, but all they could do is study the stuff around them. They couldn’t use the powerful McGuffin, which is presumably what another invading power (in this case old gods) would seek to do. Unless the assumption is N’zoth allowed himself to die so he too could study the fractals, then he’d also need to unlock the Zereth McGuffin of the void.
And I think it’s safe to assume all of the Zereth McGuffin power McGuffins need to be unlocked somehow. The First Ones are clearly bad parents, but not so bad they forgot to lock the Zereth Liquer Cabinet before going away on a holiday.
He could also do what the Devourers almost did(despite not having Sigils): bring the entire Machine crashing down!
The Cosmic Force of Life has already infected Zereth Mortis though thankfully they aren’t appropriating the programming for their use! Life seems only interested in infecting the servants of the Realms of Death and nothing else!
The Cosmic Force of the Void as well as the Cosmic Force of Light on the other hand would like nothing more to bring each other’s Machines down crashing!
Additionally, I think something that’s very interesting of Zereth Mortis that I don’t think has been fully analyzed, is its connection to Life.(And not just the water walking!)
Zereth Mortis, the inner sanctum of the realm of DEATH is full of Life. Plants, water, animals, etc…
If you remember back to the Enemy Infiltration - Preface. The Dreadlords make this remark about life:
The adherents to Life are the most insidious of opponents, perhaps because their nature is so antithetical to our own.
Zereth Mortis being tied to Life could have been an intentional safeguard to help sway the citizens of death from discovering its secrets.
We know from the Gilmore of the Shadowlands that the brokers (Firim) see the cosmos through the lens of Death, this could have been a barrier to gaining access to Zereth Mortis without the sigils.
The void’s Zereth could be similar in nature, a Light based system designed to keep the Void out.
Personally I feel that the sigils are required to enter the main structure in their respective Zereths. Which for ZM is the Sepulcher of the First Ones.