Chaeronia is a Hell-Forge that was a former Adeptus Mechanicus Forge World until it was taken into the Warp for over 1,000 standard years of subjective time, but only 100 Terran years in real-time. It is ruled by the Dark Mechanicum, although there are still a few loyal Adeptus Mechanicus troops present.
It was home to the Castigator-class Titan, an artificially intelligent, fully autonomous Titan originally created during the Age of Technology by an STC, which called itself the "Father of Titans," and claimed to be the very first Titan from which all others were but crude and poorly understood copies. As the Castigator had been corrupted by Chaos and possessed by a Daemon during its time in the Warp, it was subsequently destroyed by a Grey Knights force led by Justicar Alaric.
It is likely that Chaeronia was later destroyed by an Exterminatus order given by the Inquisition due to the extent of tech-heresy that encompassed the entire world by the time Chaeronia returned to the mortal universe.
Sources
- The Grey Knights Omnibus (Omnibus Novel) by Ben Counter, "Dark Adeptus", Ch. 7