Some of my own background for these boys; the Forge World Triplex Nicarum is a trio of worlds devoted to the creation, manufacture and use of Toxic and Rad weaponry. Hence, the planet is one gigantic nuclear-factory/toxic waste dump. What little remains of the planet's original surface is rocky and desolate, the very earth tainted by the continuous generation of poison chemicals and toxic sludge. Since the Skitarii cohorts of Triplex Nicarum are currently fending off an invasion by Hive Fleet Leviathan (The third Tyranid invasion of the Forge World in history) I have based them fighting on their own barren and poisonous world.
(Also the Alpha has been given a second pistol from the Sydonian Dragoon kit. I love the idea of a gunslinger leader for my force's biggest squad.)
The first half of Unit Sigma-Epsilon is ready for war.