Iron Collar (Fortress Worlds of the Achilus Crusade)

When the Imperium arrived in the Reach, the first phase of reconquest including establishing a firm hold over the Well of Night (the dark nebula around the Warp Gate). Six (or seven, sources vary) systems were turned into veritable fortresses, collectively known as "the Iron Collar." 

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ORPHEUS SALIENT

Hethgard (Fortress World): Site of Orpheus Salient Command. Former mining world turned into an impregnable fortress. This is where Hive Fleet Dagon was broken. It is now a dead world, being picked clean by Barotta's scavengers. The current "generation" of scavengers are a mix of fortune seekers, refugees, and criminals sentenced to hard labor.

Melkior (Fortress World): Vague references to a seventh Iron Collar world have appeared in some logs and reports. Little beyond the name is known.

ACHEROS SALIENT

Alphos (Fortress World): Former Fortress world. Alphos came under attack by Hive Fleet Dagon, but a large number of Tyranids perished due to harsh conditions. After Hethgard the Xenos continued the fight until wiped out by the Sons. Only Squats are hardy enough to live here without full protection.

Calisi (Fortress World): Former Fortress world. The cities and garrisons of Calisi were abandoned prior to the Battle of Hethgard. Fortunately, the Nids never arrived, and the planet's undesirables were spared.

Karlack (Fortress World): Former Fortress World, headquarters of Crusade Command, and chief anchorage of the Imperial Navy. Became corrupted by Chaos (Khorne). Subjected to Exterminatus (virus bombs) by order of Barotta.

Pyrathas (Fortress World): Former Fortress world. Major naval base. A colony was established here as a safe haven in case the Tyranids broken through the Iron Collar. It was (wrongly) assumed the Tyranid voidships couldn't handle the glare of the system's fierce star.

CANIS SALIENT

Spite (Fortress World): Spite was a Fortress World from which Lord Commander Sebiascor Ebongrave marshaled the forces under his command against the Tau. It was later retaken by the Tau, but abandoned as they moved on to Calixis.