The Grand Alliances are the four sweeping banners under which the peoples and powers of the Mortal Realms are grouped: Order, Chaos, Death, and Destruction. They are less formal governments than broad currents of allegiance, useful shorthand for who tends to fight beside whom in the setting's endless wars.
Order gathers the civilisations trying to rebuild the realms, from Sigmar's free cities and the Stormcast Eternals to aelven kingdoms, duardin holds such as the Fyreslayers and Kharadron Overlords, and stranger allies besides. Chaos unites the worshippers of the Dark Gods, mortal and daemonic alike, bent on corruption and conquest. Death encompasses the undead legions and underworld kingdoms answering to Nagash, who claims every soul as his property. Destruction sweeps together orruks, grots, ogors, and other rampaging peoples who fight for the sheer joy and momentum of the fight itself.
Membership in an alliance implies loose sympathy, not unity. Free cities feud with their neighbours, the Dark Gods' followers betray one another constantly, and Destruction's hordes happily brawl among themselves between invasions. The framework matters most as a map of the setting's fault lines: four irreconcilable visions of what the realms should be, colliding wherever their champions meet.