Forgotten Sons (The Horus Heresy #Short Story)

The planet Bastion must decide whether to remain loyal to the Emperor, or side with the rebel Horus. The Warmaster's case is presented by an iterator, a master of persuasion. The Imperium has sent the Space Marines. Heka'tan and Arcadese must set aside their warlike natures and learn to be diplomats, or risk losing the world. But Horus wants the world, and a plan is in motion to ensure that the Space Marines fail in their task... Read it because
Intrigue and mystery abound as two Space Marines with abandonment issues try to keep a world loyal. But something lurks in the darkness beneath the planet, and it threatens to consume them all...