Angron's Monolith (Space Marine Battles Novella)

As war engulfs Armageddon, the mysterious and mistrusted Relictors Chapter of Space Marines deploys to the equatorial jungles. As they battle the feral orks who infest that region of the shattered world, they are also scouring the jungle in search of lost battle-brothers, sent to Armageddon in search of an ancient relic with links to the world's mysterious past and to one of the Imperium's arch-traitors, the primarch Angron...

Read it because
The Relictors are an interesting Chapter, teetering on the edge of heresy as they gather Chaos-tainted artefacts to use against their enemies... and the one they're looking for on Armageddon is a doozy. After all, it's not every planet that has the remnants of an axe carried by a traitor primarch... (And if you want to see how it ended up there, you should check out The Emperor's Gift as well!)