This was.... extremely poorly written. Like, 40k in the past decade bad. Bad fanfic level bad.
I get wanting to have the Lumineth look strong. And I'm good with Nagash taking the fall in the end. But reading this, the Lumineth racking up win after win drained the ending of all tension. By the time of the Teclis v. Nagash fight, I had completely checked out because the writers had done nothing to make me think that Nagash could win, and even if he somehow did, it didn't feel like there was any danger. Everyone one of his plans had failed. His minions had been routed, his rituals had failed, he had nothing. And you never, ever, ever, ever, ever, ever, ever, ever, lessen the stakes before the climax! That's just... writing 101!
And it would have been such a simple fix too.
Make it clear that nothing will happen until all 3 realmgates are corrupted. Have Neferata and Mannfred succeed. They drive off the defenders, do what they need to do, and the realmgates are now flowing with Death Magic. If the final realmgate falls, the realms get sucked into the Shyish Nadir. Now there's urgency to the Arkhan/Eltharion battle. Eltharion blows up Arkhan, you think there's cause for celebration...
Then Nagash arrives. With Neferata and Mannfred, and a legion of the dead. Everything seems lost.
See? Right there, now things are tense! The stakes are high, if Nagash, who's a much more powerful threat than Arkhan succeeds the realms are doomed. And he has backup! And the Lumineth are depleated and exhausted from the campaign, they're in the darkest hour and need to pull through for the good of all the realms. Isn't that so much better than Pointy Boiz win lolz?