This archive tracks four layers. A record present in 3 of them (HH, M41, IND) is silent in the rest, and says so.
The Horus HeresyM30–M31
chaos
Site of the Isstvan atrocities
The Drop Site Massacre. Seven legions landed to bring the Warmaster to heel and four of them changed sides on the ground; the Urgall Depression is where the loyalist Legions died.
Age of the ImperiumM32–M40
[RECORD LOST]
No record for this location in Age of the Imperium.
The 41st Millenniumpre-Rift
none
Dead world; interdicted
Uninhabited and shunned. Twelve billion people lived here before the Heresy.
Era Indomituspost-Rift
noneImperium Sanctus
Dead world; interdicted
Nothing has changed and nothing is likely to.
Position
Map frame: 640, 440 in the archive's 0–1000 disk space (core at 500, 500).
Distance from the core: 152 units ≈ 16.2k light years, north-east of the galactic core.
Height above the disk plane: 3.7 units — invented thin-disk jitter, deterministic per record. No published source gives any location a z coordinate.
wh40k.lexicanum.com/wiki/Isstvan_III infobox Class = Dead World; Population = None (was 12 billion pre-Heresy). Isstvan V's own infobox leaves Class blank, so it inherits the system's state
gposlow
derived: Isstvan system, Ultima Segmentum, chosen by the Warmaster as remote from Terra but astride his line of advance — placed in western Ultima on the approach from the eastern crusade fronts toward the Solar segmentum. No published bearing exists