The Celestial Engine was not on the front lines.
As a support weapon assigned by the main fleet to the White Scars splinter fleet, the Celestial Engine had always been used as an exceptionally useful spaceport and supply station.
In normal combat environments, the Solar Engine within the Celestial Engine could destroy several star systems across light-year-scale distances, but its usefulness was rather limited within the Eye of Terror.
It could not be used against daemon worlds where a few sorcerers could turn orbital bombardment into fireworks with a ritual, and it was unnecessary after Blackstone had been deployed to occupy an entire world.
Fortunately, this gigantic void Construct served as excellent logistical support in its struggle against Warp power. This meant that no matter how quickly the White Scars advanced, they never had to worry about their lines becoming overstretched.
When the deployment order was issued, the core of the Celestial Engine's planetary structure opened, and the Hypercube Maze containing and draining a fragment of the Nightbringer was extracted.
The Hypercube Maze was placed directly into a device capable of trans-dimensional jumps, advancing through the realspace corridors carved through the Eye of Terror by vast amounts of Blackstone, beyond observation or interception.
The landing force attacking Medrengard used the Lightning's devices to teleport out, leaving behind only a fortified position at the landing site. Still unsure why the enemy had suddenly fled, the Iron Warriors surged into the position and cleared away the various fortifications they considered utterly inferior.
The Blackstone Constructs deployed to the surface alongside the landing force were dismantled one after another.
The enormous Blackstone Constructs deployed only after the landing force had withdrawn were dealt with by large war machines such as Titans.
These unattended black things posed no threat without friendly protection. Destroying them