New book's rules are mostly in the scenario sense, and don't add much for us. There's some background, but it's pretty unclear, and it doesn't name anyone for our team other than Nagash.
Here's a brief summary of the history of the Undead from the End Times to the modern Age of Sigmar according to the book:
-----------------------------------
After the destruction of the old world, Nagash found his consciousness had survived the end, coalescing within the death magic saturating the new splinter realm of shyish. Many of the various undeworlds and minor death gods from various old world cultures also remained, and Nagash spent the following age conquering each underworld in turn and consuming these rival deities. By the time Sigmar forged his grand alliance, Nagash stood supreme as the sole god of Death and ruler of all underworlds.
During the age of peace between the Gods, Nagash tolerated the existence of the other Gods and other realms, taking his place in the pantheon as god of the dead. The spirits of the dead of all worlds eventually passed into his keeping, and though he may have plotted usurpation, he did not act against the other realms, and even tolerated communities and cultures of the living within his own realm, so long as they worshiped him.
Then came the war of chaos. Where the other realms were assaulted by armies of daemons and mortal warriors, Nagash's realm was first set upon by the skaven, gnawing their way threw the barriers between realities to infiltrate Shyish from below. These attacks left Nagash's forces considerably weakened and unable to commit themselves fully to the greater war effort. Though it isn't explicitly stated from what I can tell, to the rest of the alliance it must have looked like Nagash was holding the bulk of his forces in reserve, waiting to see which side would gain the upper hand.
During a key battle at the height of the war on chaos, the undead suddenly switched sides. The book doesn't overtly state what happened (imo it has the stink of Mannfred's duplicity all over it), but Sigmar assumed betrayal by Nagash, and pulled out of the battle to go after the Great Necromancer in what became known as 'the wars of heaven and the underworlds'. They fought several times, with Nagash always escaping, but by the time Sigmar finally returned to the real fight it was too late to turn the tide. At the same time, the clashes with Sigmar left Nagash so weakened that Archaon was able to best him in single combat when the forces of Chaos finally arrived in Shyish (or however its spelled). Nagash's surviving mortarchs (no mention of who specifically, but one has to imagine Arkhan was the key actor here), were able to retrieve the remains and retreat to the deepest underworlds, where chaos was unable to follow and even the skaven couldn't penetrate.
Eventually Nagash's spirit coalesced and returned to his body, and he lurked in his underworld soaking up power until he was able to stage one of his signature comebacks, with ghostly armies surging from the underworld and necromantic magic saturating his realm such that all dead would rise in Nagash's service. The occupying chaos forces were completely overwhelmed - a powerful chaos champion might hack apart 50 zombies before being dragged down by weight of numbers, only for all 50 zombies to pull themselves together and rise back up after, joined by the champion's corpse. However, renewed skaven assaults from below once again stall Nagash's advances, and it seems the Great Necromancer is doomed to be forever stymied by the plots of the rat men, whether in the old world or the new.
And that takes us to the 'present day' in Age of Sigmar as far as Nagash and his realm is concerned. Though Nagash and Sigmar remain enemies (all the more so, now that Sigmar is claiming the souls of slain heroes to fill out the ranks of his new celestial army), the Sigmarines have been mentioned as showing up in support of the undead battling against the forces of chaos.
--------------------------------
The bulk of the 'present day' fluff and narrative scenarios in the book focus on the sigmarines working to reclaim realm gates from the forces of khorne in the realm of fire, and supporting Alariel's dryads and tree-types in the realm of Life against a renewed incursion by the forces of Nurgle, though there are some non-faction-specific, non-realm-specific scenarios that work with us. There's a 'stop the ritual' scenario that is a particularly good thematic fit for us.
I assume that there will eventually be some campaign expansion focusing on the 'wars of the dead', expanding our cast of characters somewhat and presumably bringing new scenarios and maybe models for undead and skaven factions in particular, but from the sounds of it such an expansion won't be showing up any time soon. Rumors are that we've got a few more weeks of Sigmarines, and then Khorne, and after that maybe something 40k, and then back to Age of Sigmar with some Tzeentch related campaign expansion (plus maybe a tie in for 40k), and that should already have us into next year, so yeah, I don't expect anything new on the Death front any time soon.
That said, we do have a lot to work with just with the VC and TK rules we've already got, so... *shrug*.
