Overview
Ullanor Prime was the throneworld of a great Ork empire until the Emperor's Great Crusade broke it, and the victory was crowned by the Ullanor Triumph — the high-water mark of Imperial glory, where Horus was raised to Warmaster before the assembled Legions.
It is, by canon, the same world as Armageddon. After the War of the Beast in M32 the Adeptus Mechanicus used captured Ork teleport technology to move the stripped Ullanor Prime to the edge of Segmentum Solar, faked its destruction, and left it to be re-colonised under a new name.
This marker therefore stands at Ullanor's historical position; its segmentum entry records where the crusade histories placed the world, not where the living world of Armageddon now lies.
M41 and M42 status
Historic site of the Ullanor Crusade and Triumph.
Canonically the same world as Armageddon — teleported and re-colonised long ago.
Timeline
The Great Crusade breaks the Ork empire of Ullanor; the Ullanor Triumph crowns the victory and raises Horus to Warmaster.
The War of the Beast: a second Ork empire rises on Ullanor and is destroyed after bitter fighting.
The Mechanicus teleport the stripped world to the edge of Segmentum Solar and record it destroyed; it is later re-colonised as Armageddon.
Eastern campaign planners still cite the old Ullanor position as a symbolic military reference.
The archive keeps Ullanor marked as history — a triumph whose world now answers to another name.
Cartographic positions and several administrative details are intentionally approximate where published Warhammer sources conflict or provide no canonical coordinate system.