This archive tracks four layers. A record present in 4 of them (HH, AoI, M41, IND) is silent in the rest, and says so.
The Horus HeresyM30–M31
imperium
Muster world; the Battle of Calth
Ultramar's second world and the assembly point for the Legion's march on the Warmaster — until the Word Bearers turned on the fleet in orbit and burned the surface. The star itself was poisoned.
Age of the ImperiumM32–M40
imperium
Subterranean world
The surviving population lives under the crust in arcologies, hunting the Word Bearers who stayed behind in the tunnels.
The 41st Millenniumpre-Rift
imperium
Subterranean world; the Underworld War
Still inhabited, still underground, still fighting the same war in the caverns ten thousand years later.
Era Indomituspost-Rift
imperiumImperium Sanctus
Subterranean world
A working world of Ultramar again in every way except that nobody can stand on its surface.
Position
Map frame: 684, 552 in the archive's 0–1000 disk space (core at 500, 500).
Distance from the core: 191 units ≈ 20.3k light years, east of the galactic core.
Height above the disk plane: -6.5 units — invented thin-disk jitter, deterministic per record. No published source gives any location a z coordinate.
System: Veridian System
Books set here
8 titles in the reading tracker touch this record, 2 of them as a primary location. Titles link out to the tracker.
wh40k.lexicanum.com/wiki/Calth infobox Class = Civilised World, Death World; rendered dead for the scoured surface that dominates three of its four eras
gposmedium
derived: Veridia System, Ultramar subsector — placed immediately alongside Macragge. The visible separation is for label legibility, not scale