Inwit Cluster

The Inwit Cluster is a tightly packed ice-world group in Segmentum Tempestus, also called the Inwit System, from which Rogal Dorn ruled a small empire before the Emperor found him. Inwit itself is hive-caverns under kilometres of ice, nomad clans, and shipyards that later lore puts on a par with Jupiter.

Dorn left; the Imperial Fists kept recruiting here. Horus's war then spent four years trying to crack the cluster, and an Emperor's Children-led traitor fleet burned a string of its worlds before the Siege of Terra pulled the traitors away. The Feast of Blades still sometimes comes home to this ice.

The Inwit Cluster still matters as Dorn's other throne besides Terra. Unofficial fan maps keep the segmentum-tempestus parent and the sector-tier pin; this is a cluster, not a single anonymous iceball.