By the Lion's Command (The Horus Heresy Short Story)

Following the splitting of the Dark Angels fleet in the aftermath of Perditus, Seneschal Corswain continues to hunt down the Death Guard forces under the command of the infamous Typhon. Now, a tense standoff erupts in the heavens over the supposedly independent world of Terra Nullius, and Corswain must either bend the local population to his own cause or make an example of them to other worlds that might secede from the Imperium. In this war, there can be no innocent bystanders. Read it because
It's a tense stalemate from start to finish. An ultimatum is set for the Free Army of Terra Nullius by the Dark Angels. They can rejoin the forces of the Emperor or be destroyed along with the traitor Death Guard. This is one of the first instances where a system declares independence rather than joining either side in the Horus Heresy, forcing the Loyalists to make some hard decisions about them.