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
Homeworld of Magnus and the XV Legion; burned
Tizca, the city of glass and marble, held the greatest library the Imperium ever assembled. The Space Wolves and the Custodes burned it in retribution for Magnus's sorcery, and the survivors were pulled bodily into the warp.
Age of the ImperiumM32–M40
none
Dead world
The ruin of Tizca under an empty sky; nothing lives on Prospero for the next ten thousand years.
The 41st Millenniumpre-Rift
none
Dead world
Still a grave, still visited by those who want what the Thousand Sons left behind.
Era Indomituspost-Rift
chaosImperium Sanctus
Reclaimed by the Thousand Sons
With Magnus returned to realspace the Thousand Sons came back to Prospero in M42 and began repopulating it as part of an empire inside Imperial space.
Position
Map frame: 596, 385 in the archive's 0–1000 disk space (core at 500, 500).
Distance from the core: 150 units ≈ 15.9k light years, north-east of the galactic core.
Height above the disk plane: 2.6 units — invented thin-disk jitter, deterministic per record. No published source gives any location a z coordinate.
derived: north-western Ultima Segmentum, close to the Obscurus/Solar junction and south of the Rift line. Published sources give the segmentum and sector but no intra-segmentum bearing, so this is an editorial placement