In what should have been the pinnacle of his career, a lone remembrancer meets an unfortunate end at the hands of the Luna Wolves. So close to the heart of the Warmaster's emerging powerbase, were the signs of Horus' fall apparent from the beginning, yet heeded by no one?

Read it because
It gives an insight into why the Remembrancer order was created – and how, and by whom – and acts as a prequel of sorts to Horus Rising, with a guest appearance by the heroic, and ill-fated, Hastur Sejanus.