Savage Weapons (The Horus Heresy #Short Story)

For two years, the Dark Angels have battled the Night Lords on the worlds of the Aegis sector, and they are no closer to victory. When the traitor primarch known as the Night Haunter issues a challenge, Lion El'Jonson of the Dark Angels descends to an abandoned world for a final confrontation with his treacherous brother. But the Night Haunter has a darker purpose...

Read it because
It's a clash of primarchs, as the Lion takes on Konrad Curze (and not for the last time). It's also the first appearance of a certain Jago Sevetarion, who will go on to become pretty important to the Heresy.