Looking back at the lord-octopus of Northrend with the gift of hindsight, I can’t help but wonder about his motivations. For an Old God of the Void Lords he seemed really fixated on death. Some of his spells were death-related (even his Hearthstobe effect). He had the blue-green hue common of death-related individuals in Blizzard IPs. He talked on and on about death in the visions of his mind; the death of the blue dragons, the way that deaths like King Llane’s from faction wars somehow feed him, and the seeming inevitability of the death of any king — a sentiment Terenas eerily echoed to his son when his soul was released from the Nathrezim blade.
At the throat of the world since the days of the Azerothian Pangea and seemingly adjacent to whatever portal to the Shadowlands that Icecrown seems to be, Yogg would presumably have the closest connection and insight to “the enemy of all” (Death) of any of the Void-allied. His quote “only death is eternal” eerily echoes Sylvanas at Vol’jin’s funeral, almost sounding like a dejected abandonment of any hope to ever achieve the designs of his creators (perhaps seeing even the Void Lords as kings who can’t rule forever and must ultimately kneel to Death.) He talks about “the next [life]” and how there is no escape even in death.
His puzzle box even mentions sleep and how nothing breathes (is alive?) beneath the giant rook’s shadow. Not unlike the tall-shadowed Mueh’zala (the Father of Sleep), Yogg (“the Lucid Dream”) shapeshifts throughout the fight. He was responsible for the Curse of Flesh that would age mortals and make them more vulnerable to death, as well as the corruption of the Emerald Dream — echoing Sylvanas’ essentially destroying a world tree and making herself “the enemy of all life.” And if Mueh’zala was the creature who bargained with Odyn then from his point of view he got a great deal when Yogg-Saron twisted Helya to seal the Prime Designate away so that he couldn’t use his powers from the Shadowlands to interfere with Mueh’zala’s plans (until recently.) Even Yogg’s chosen form of the levitating female Vrykul Sara seems tied to Val’kyr, who are tied to Helya and Odyn, who’s power to make Val’kyr is (probably) tied to Mueh’zala.
So did Yogg “strike a bargain with the enemy of all” as the Light had? Assuming Blizz had all of this clearly hashed out 12 years ago seems far-fetched (or even earlier considering Arthas’ journey through Azjol’nerub), but it is something they could retroactively explain without retcon. I’m honestly not sure what any of these correlations would even mean going forward with Yogg dead and all, but as he died he seemed pretty certain that our fates were sealed anyway. He seems to view the Void Lords’ “seeding” of Azeroth as irrelevant in the face of unavoidable annihilation and Death’s ultimate victory. And his corpse will supposedly “choke this land”, the “throat of the world” as Malfurion called Northrend. And if Yogg-Saron’s spirit passes on to the Shadowlands with the collective amassed psychic energy or whatever he meant by “a thousand deaths or one murder... only [making him] stronger” (thinking of doubt, pride, etc. feeding the Sha/Y’Sharaj in Pandaria) then that could potentially be a major source of power for Mueh’zala to feed on; as would Yogg’s siblings — which is possibly the fate the Titans temporarily avoided by imprisoning them. But this isn’t the only explanation.
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Also meant to mention another tie between Yogg and Sylvanas with Saronite being the only thing capable of granting her banshee soul the true death in Edge of Knight. And to mention that Frostmourne was created by the Nathrezim who also dabbled in death and void magic with Old God knowledge, and that Primordial Saronite was instrumental in forging Shadowmourne.