The secrets in the Liber Mortis are not common knowledge but well...
Vlad was there every step of the way. He is the cowriter of the Liber Mortis, wrote the conclusion and everything.
Went on to found an entire Vampire Bloodline, which he taught Necromancy to.
Even if the full text is lost to many of them, they cannot be that secret.
And while the first vampire war was terrifying, He felt the need to use vampires and human auxiliaries to pad out his troops.
He still lost a lot of battles and even the full war in the end.
Mathilde hyped up how with those secrets she would become an unstoppably powerful necromancer if she tried. And that might well be true.
But working with Dhar is hard, and it is unstable. It often blows up in your face. There is a lot of room between safer than mainlining pure Dhar, and actually safe.
Going from the theoretical to the practical here is likely a significant hurdle.
If you assume all the strengths are open to anyone, and all the drawbacks are because Van Hal wasn't a very good mage then it is an unstoppable doom weapon.
Mahilde while reading through it regularly alternates by mentioning how little talent Van Hal has, and how they are the innermost thoughts of a genius.
Vlad Von Carstein took one look at his work, and said if he couldn't do it, testing more humans was pointless.
Vlad knew Nagash personally.
Having access to both does not make you the next Frederick Van Hal.
The internal dialogue on Frederick Van Hal is less of a contradiction than you might think, mostly because you are taking quotes way out of context
Van Hall had little innate talent for the arcane, his genius lay in his more mundane military acumen and his ability to take what little gift for magic he did have and wring it for all it was worth under the tutalage of Vlad von Carstein
Vladimir didn't just offer a book to Frederick, but remained at his side and walked him past every one of a thousand traps and triggers that it contained and helped him parse the few diamonds of wisdom hidden in a sea of knives. Frederick seemed to have no more talent with magic than most, but what little he did have he cultivated to handle Shyish, which had been his constant companion since the Plague first arrived in Sylvania. And when that tiny ember of potential grew to its full potential, which was barely that of a smouldering slow-match, he took it and pressed it to the readily-available gunpowder of Dhar.
Over nearly a decade, the undead and the Skaven fought to a standstill in a type of war unlike any you've ever known, where lives are expended like munitions and arbitrary amounts of reinforcements are a given. Dozens of tricks, traps and feints are laid out in exhaustive detail, as are the means Frederick used to counter them. You're able to use these chapters and your ruminations on them to identify and examine a great deal of your own assumptions about warfare because of how thoroughly Frederick's war shatters each of them. And throughout it all, detailed every month like clockwork, you see a genius track his own progress towards incurable madness, as the cumulative exposure to Dhar begins to show. And to your surprise, the notes are supplemented by the incredibly neat and precise handwriting of what must be Vlad who is not yet von Carstein, as he gives an external and impartial perspective on the progression of the corruption.
The nature of Dhar is that it is potent but unstable, which is why petty sorcerers with such limited educations on the nature of magic can pose such a real threat to trained magisters who've dedicated their lives to refining their art
And why it's so tempting, it's a lot power on demand for any caster who's just willing to reach out and grasp it, without having to rely on years of careful study and practice
It's easy and readily available power that you don't need a lot of innate talent to wield
It's also unstable and corruptive of course, which are the primary reasons not to give into temptation, I'd wager a significant chunk of would be sorcerous overlords have blown themselves apart, crumbled under mutations, or otherwise taken themselves out of the equation when it turned out that that they could grasp power, but couldn't properly control it
Casting with Dhar is a gamble because its chaotic nature means that there is always a far greater possibility it will break free of your will and wreak havoc, it's a game you will lose eventually
The First Secret of Dhar solves a big problem here, instead of having to simply grab onto Dhar and struggle to force it to obey your will you bind it upon itself
You force stability and structure onto it while retaining its power
You gain the reins by which you can hold onto to Dhar and weave it into structured spells rather than having to haphazardly dip your hands in and mash it into shape before throwing it at the target
It's not actually obvious, you concede. It's actually entirely counter-intuitive and if it hadn't been broken down for you by the writings of a long-dead genius you'd never have believed it could work that way. Dhar is inherently unstable, of course. So you use as little as possible, as quickly as possible, right? So it has the least amount of power and the least amount of time to break free?
No. With the patience of a priest, you weave it atop itself again and again like the cords of a rope, with every strand of it straining with the desire to explode free but held in place by every other strand. And just like that you have all the power of Dhar and none of the drawbacks... at least, not unless your attention wavers while attempting it and you burn alive from your soul outwards.
Of course if you ever do lose focus and the structure unwinds it'll blow up in your face, but simply having a proven method by which you can make Dhar act in a stable manner so long as you don't screw up is a huge improvement on wrestling it like a rabid animal
And it means you can handle vastly greater amounts of Dhar, greatly amplifying the power you can wield without needing a greater innate connection to magic
The counterintuitive nature of this knowledge makes it especially unlikely for someone to reach it on their own without guidance, and the advantage it gives makes it unlikely that many would be willing to share such knowledge if they possess it
Especially given how Dhar exposure tends to cause maddening paranoia among its many pleasant effects
Likewise you take the Vlad dismissing the idea of anyone managing to wield Nagash's art safely if Van Hall couldn't way out of context
Vlad was talking about his moral character, not his talent for wielding magic
Despite what safeguards Vlad could provide Van Hal's limited exposure to Dhar still corrupted his mind and drove what was once a good man to madness
Leading Vlad to conclude that mortals simply
cannot wield necromancy safely
With his sanity fraying and the Skaven escalating their presence, he begins to take apprentices from among his subjects, and with so few remaining and so few of those with even the tiny sliver of magical ability necromancy required, he couldn't be choosy. Much of the pages are consumed with him doubting their loyalty or motivations, and then him examining his own doubts to try to decide if they're genuine or further signs of his mental degradation. The neat hand of Vlad appears more and more, chronicling events that Frederick never got around to mentioning, having instead filled the pages with his second-guessing of himself.
Even as the war against the Skaven begins to swing in Frederick's favour, the war within his mind spirals quickly. By the time that the Empire had mobilized under the future Emperor Mandred, Frederick was so far gone that with three pages of scrawled suspicion and paranoia he had talked himself into believing that they were allied with the rats and had begun to formulate battle plans along a second front. The rantings are cut off mid-sentence, and only a single line appears below:
The results are conclusive. Nagash's Art cannot be safely wielded by even the best of mortals. May his Morr be more merciful than Usirian.