I think it's possibble that the magic isn't being stored for any future event, but might be being actively used to make sure that the Vlag-Be-Gone spell sustains itself. For instance, perhaps rather than simply vanishing in one super-huge ritual, whatever happened here instead draws power from the waystone to keep Vlag vanished. And if this flow were somehow interrupted Vlag would return to existance.
That was one of my thoughts, yeah.
And even if not, if this has been going to over 200 years by now, I doubt they're going to waste their accumulated bounty on our expedition. More likely they're saving it for something specific, or a much bigger opportunity than this. Only reason they would use it now would be if they realize we've figured things out and it becomes a use it or lose it situation.
...
...
... Maybe they're using it to allow the Chaos Wastes to go as far south as to overtake Karag Dum? (My original guess was actually just going to be "Maybe it's being used to put Karag Dum in the time-slow
" but then I thought
'Wait. What if it's the Chaos Waste itself being able to reach further down south?')
Maybe
that is where the energy is going?
Maybe
that is how the Chaos Wastes are able to overtake and gain territory?
By lite
rally taking over the territory that the waystone network was on, and redirecting the energy to the North instead of to the Vortex?
This could potentially be big. Bigger than 'just' a 'Big daemon portal?' or 'Missing Karak?' If it's actually responsible for the Chaos Wastes being able to encroach further south into the world, by conquering and taking over parts of the Dwarf Waystone Network, then...
Karag Dum and Karak Vlag might not just be a huge deal for the Karaz Ankor, by restoring more power to the great Karak-runes of Valaya's protection. They might also be a huge deal for the whole world, too. The Dwarfs of Karag Dum might have been right all along, about Chaos trying something huge; not just an Everchosen invasion, but an invasion of the Chaos Wastes themselves a little bit further down south. The Everchosen failed, or at least did not succeed as much as they hoped for, but maybe they got at least some benefit out of it, in the form of 1 or maybe 2 parts of the Waystone Network being captured or subverted.
Maybe Karag Dum is still hanging on. While Karak Vlag was taken, and its waystone network energy being redirected. If so, Karag Dum falling and finally losing the Hammer of Fate and the Runemaster's Axe would be disastrous for the future of the world.
Perhaps that's even why Karag Dum held out where Karak Vlag did not. Because of having the Axe and Hammer.