DEtherocBalNAzzarVarimaTHRasTichondrIUSAdd the capitalized letters together and you get DENATHRIUS. Probably a huge coincidence(wink wink nudge nudge), but cool nonetheless.
The location of the book does have bearing as the Nathrezim call themselves "unseen guests" and the tower is called "Spire of the Unseen guests".
So I would argue Varimathras didn't fake killing Balnazzar and took it as a chance to get close to Sylvanas. Once he figured he couldn't control her, he staged the coup with Putress to try and kill her. His punishment in Antorus seemed particularly cruel and I think the other Nathrezim suggested this punishment as he broke what I suspect is one of their two laws. This one of two being it is forbidden to strike another Nathrezim down... and for his failure in destroying Sylvanas. Why would his first words to the raiders be about her if he didn't hold such animosity towards her? His whole gambit was to get close to Sylvanas and control her which required a great sacrifice on his end. He might have gotten off with a less severe punishment if he was able to remove Sylvanas from the equation. Goroth is the perfect example of normally failing Sargeras. His form was still intact but he was still mutilated while Varimathras was literally being butchered. His jaw ripped and forced open, eyes gauged out, horns broken, wings shredded, skin flayed and metal spikes inserted. Possibly killing Balnazzar was a feint, but his failure to summon which was assumed Sargeras into the Undercity doesn't seem like such a catastrophic thing to warrant such punishment. The Nathrezim most likely suggested it to keep their hands clean. It would have undermined the Legion authority structure and put a spot light on them if they did it. The time doesn't fit this crime and this is assuming the Coven was uniquely capable of inflicting permanent damageMaybe Varimathras did betray his brethren to get close and stop Sylvanas because he didn't want her to help the Jailer. It isn't an uncommon trope in fiction for an evil agent to be tired of the evil. Maybe the Nathrezim's origial modus operandi was to simply learn all they could about mortal races and record it, but the Jailer twisted it and made them just want to make mortals suffer and record the interesting and unique ways mortals could be pushed to commit atrocities. As if the injustice visited upon the Jailer was so great... WHAT IF the Jailer was the original book keep of creation and the Nathrezim his custodians. The Maw would in fact be a library with out end which would in turn make the perfect jail! Nathreza being a temporary library until the Jailer was freed and yes it is still probably fully intact after Illidan's excursion. I should be asleep....
I do think people should be more careful with Il'gynoth whispers, they more than often are deceptive, and when something seems to be "perfectly fit", it often isn't...
Sire Denathrius... sired the nathrazim... how is it not that obvious to everyone? It's literally in his name.
I think Il'gynoth referenced that it's the Nathrezim, as one of its quotes is 'The cunning ones kneel before six masters, but serve only one'.
The nathrezim not really dying really ties in with what we see in the game. I have always thought it was interesting how Dreadlords Die: they just disappear. In WC3, they would turn into a bunch of bats but in WoW they juts are gone and leave their armor behind so it could very well be that they just go back to their native realm of the shadowlands and then keep going with their manipulation game.