When the campaign heats up, it will involve a "Zombie Apocalypse" and the general collapse of society. The concept of a "zombie" plague in a game universe which spans infinite worlds that are connected (and isolated) from each other in unconventional ways is a neat one, and will be explored. To say more here would be spoilers, so I'll update this section as the campaign progresses with a group of players representing the "main characters" (those characters may die and find themselves a rotating cast).
Note that, as this is a fantasy setting, the Undead are not unheard-of already. The "zombies" which form the faction responsible for The Crisis will be custom-made according to a series of templates, and are distinct from the reanimated corpses a player's Necromancer character might be able to control. Nevertheless, this is a relatively low-magic fantasy setting. Magic is rare, and magic-users treated with suspicion by most. Summoning the dead would be a surefire way to get in trouble in the Kingdom of Nool, and probably cause a serious incident. All of this to say that there is nothing stopping players from commanding or encountering more traditional undead before, during, and after The Crisis.
As I am looking forward to simulating things here, I have gone out of my way to create interesting and asymmetrical connections between all the population centers in the game universe. Regardless of how the campaign itself goes, simulating the "zombie apocalypse" will be very neat over time!
The over-arching plot will rely on background events which are heavily simulated between sessions, based on what the players do or don't do and the state of the game world as they've influenced it. How the player party decides to handle strategic decisions once the campaign is in full-swing will radically change the way the simulations go, in ways hard to predict. There will ideally be an XCOM-like turning of the tide against the "big bad" (whomever that may turn out to be) if the players are up to the challenge. It would be poor GMing to rely wholly on the dice, but I will be letting the systems take their course, for the most part, once things reach that stage and the players are comfortable enough with their characters to begin the main challenges in the campaign. Players shouldn't worry too much about being killed outright (which is always possible but I'll strive to prevent that within certain limits), but they should worry about the way the game state will evolve over time if they aren't trying to tackle the strategic challenges (and that may result in a "lost campaign" where the players are forced to flee Nool entirely, if it doesn't result in their characters being slain in battle).