The first short story in the collection is Rules of Engagement by Graham McNeill, which takes place in Ultramar at the height of the Heresy. Ultramarine Captain Remus Ventanus, hero of the Battle of Calth, leads his 4th Company in a series of battles across the Five-Hundred Worlds, battling Death Guard on Talassar, World Eaters on Prandium and Salamanders on Quintarn. Only when Macragge itself falls to the Sons of Horus is it revealed that all of the battles were fake simulations carried out in order to test primarch Guilliman's new battlefield omnibus, the Codex Astartes. This short story offers a glimpse of things to come, not only introducing Guilliman's paradigm-altering publication but also containing the first mention of the Imperium Secundus.
Liar's Due by James Swallow is set on the insignificant backwater world of Virger-Mos II, and explores the consequences of Town 44 receiving false news that Horus has slain the Emperor and now rules the galaxy, thanks to the manipulations of an Alpha Legion agent. This short story is actually a 30K take on a Twilight Zone episode, The Monsters Are Due on Maple Street, in which the people of a quiet American town are manipulated into turning on each other by the mere threat of outside attack. Liar's Due is an intelligent short that shows us a refreshingly different but disturbing side of the Heresy.
Forgotten Sons by Nick Kyme follows two loyalist marines, Arcadese of the Ultramarines and Heka'tan of the Salamanders, as they advocate for the Imperium on the neutral world of Bastion that has yet to decide its allegiance. Two separate traitor plots ensure that this ends in disaster, and despite the marines' heroic efforts Bastion is destroyed as a warning to other worlds considering siding with the Imperium. This short story doesn't have much to offer other than its average plot.
The Last Remembrancer by John French follows Rogal Dorn, primarch of the Imperial Fists, and Iacton Qruze, agent of the Sigillite, as they decide the fate of Solomon Voss, a famed remembrancer who has spent the war at Horus's side. The shortest of the nine stories, its main theme is the pain Dorn suffers upon coming to realise that the ideals of the Imperium have been destroyed by the dark necessities of the war, necessities like his executing of Solomon Voss.
Rebirth by Chris Wraight follows Captain Menes Kalliston of the Thousand Sons as he return to Prospero to see what has become of it after the Space Wolves' invasion. Captured by Kharn of the World Eaters, who have come to Prospero on a mission from Horus, Kalliston learns of the Heresy and offers to heal Kharn's mind but is beaten to death for his trouble. This promising debut from Chris Wraight has more depth than your average short story and enticingly hints at a link between a survivor from Kalliston's squad and the origins of the Blood Ravens.
The Face of Treachery is Gav Thorpe's follow-up from his audio drama Raven's Flight, adding a World Eaters ship captain determined to attack the rescue fleet into the narrative of Corax and the surviving Raven Guard's escape from Isstvan V. This story is mostly just fleet maneuvering, but segues nicely into Thorpe's first Heresy novel Deliverance Lost by revealing the Alpha Legion's infiltration of the Raven Guard.
Little Horus by Dan Abnett brings the series back to the Sons of Horus for the first time since Galaxy in Flames, in the form of an attack on the planet Dwell led by 'Little' Horus Aximand who is struggling with unnerving dreams. Little Horus packs action, great writing and universe expansion into a small word count. Here we find the first appearance of characters Shadrak Meduson and Hibou Khan, making this short story a crux around which later writings will revolve, and the final lines in particular are masterful.
The Iron Within by Rob Sanders is a siege story set in the catacombs of Lesser Damantyne, where loyalist Iron Warrior Warmsith Barabas Dantioch resists attempts by his traitor brethren to overcome the Schadenhold, a fortress built into a giant stalactite hanging from a cavern ceiling. While there is no shortage of action the best feature of this short story is Dantioch himself, a superb character who calmly remains one step ahead of his foes the entire way. The Iron Within is a promising start for Rob Sanders.
Savage Weapons by Aaron Dembski-Bowden introduces us to the Thramas Crusade for the first time, where Dark Angels and Night Lords battle it out across the sector. The story is simply a meeting between Lion El'Jonson and Konrad Curze on Tsagualsa that predictably goes sour, but Dembski-Bowden's writing makes it entertaining regardless, as does the presence of everyone's favourite Night Lord Sevatar. Savage Weapons' true value is that it introduces a corner of the universe that has been revisited several times since.
As an overall anthology Age of Darkness is hard to judge; some of its stories are good, the best being Little Horus, and some not so good, the least meritorious being Forgotten Sons. The biggest difficulty in judging it comes from the fact that all the shorts have different purposes: some throw out plot threads that are not picked up again until much later (Rules of Engagement, Rebirth, Little Horus, The Iron Within), some are setup for following novels (The Face of Treachery) and some are simply short stories (Liar's Due, Forgotten Sons, Savage Weapons). Overall the quality is good, but Age of Darkness is more a mosaic than anything else.