Dark Imperium, published in 2017 by Guy Haley, is the flagship novel for the setting's dramatic reset following the opening of the Great Rift, and it shoulders that role by placing one of Warhammer 40,000's most consequential figures at its centre: Roboute Guilliman, primarch of the Ultramarines, dragged back from ten thousand years of stasis to find his father's Imperium transformed into something he barely recognises.
The premise is rich with dramatic irony. Guilliman was a rationalist and a builder, an architect of law and civilisation in the age of the Great Crusade. He returns to a galaxy that has made his absent father into a literal god, where superstition and despair have replaced reason, and where the very institutions he designed have calcified into cruelty. Haley mines this dissonance for the novel's most interesting material: the primarch is at once the Imperium's greatest asset and its most dangerous potential heretic, because he can see with clear eyes just how far mankind has fallen, and he is not sure the thing he has been resurrected to save is worth saving in its present form.
The immediate crisis is military. Guilliman's home realm of Ultramar is engulfed by the Plague Wars, an invasion of pestilence and daemonic corruption led by the Death Guard and their primarch Mortarion, once Guilliman's brother, now a bloated servant of the Plague God. The war between the two primarchs gives the book its spine, and Haley threads it with the venomous history of siblings who chose opposite sides of the setting's defining betrayal. Interludes among the corrupted, and glimpses of the daemon primarch's court, keep the enemy from being a faceless tide.
Haley balances the grand scale with the practical business of command. Much of the novel concerns the sheer difficulty of waging a modern Imperial war, the logistics, the politics, the friction between Guilliman and the priesthood, and the arrival of the Primaris Space Marines, the new and larger warriors who accompany his crusade. For readers coming to the setting after its edition change, the book effectively narrates why the galaxy now looks the way it does.
What lifts Dark Imperium above a simple relaunch is its melancholy intelligence. Guilliman is written as a weary, honourable, deeply isolated figure, burdened by knowledge he cannot share, and the Plague Wars around him are genuinely grotesque in the Nurgle tradition. It is both a war novel and a meditation on faith, disillusionment, and the loneliness of the one man clear-sighted enough to grieve for what the Imperium has become. As an entry point into the current era of the setting, it is close to essential.
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