Distant Echoes of Old Night (The Horus Heresy Short Story)

Death Guard Chaplain Murnau has shot down an Imperial Fists vessel on a dismal backwater world, and the forces under his command are closing in on the wreckage. With his mission clear in his mind to leave no survivors, Murnau unleashes one of his Legion's most deadly units the fearsome and infamous Destroyers. Read it because
It’s the first Horus Heresy appearance of the Destroyers – warriors who use forbidden weapons and deeply dubious tactics to not just defeat their enemies, but obliterate them.