The Echoing Vault is a mysterious artifact.
In M32 the artifact was uncovered by an Explorator fleet deep within the Halo Stars. Its origin is unknown, but it quickly unleashed a wave of horrors never seen before by the Imperium. These xenoforms were dubbed "The Harrowing" and violated the laws of physics, causing those even within their proximity to die or be driven insane. They ravaged all in their path for several years, creating unprecedented losses for the Adeptus Astartes and Inquisition as well as other Xenos factions.
Ultimately, it is said that an Inquisition faction found that the creatures could be contained with a fusion of archaeotech and Sorcerous lore. Defeated, the creatures fled back to the Echoing Vault and the Mechanicum used a Dark Age-era forbidden weapon to seal the breach the device created.
The Echoing Vault was so terrible a device, and its method of defeat so heretical, that in the aftermath of the conflict all records of it were purged from Imperial history. Nonetheless the Vault is a subject of fascination for several Radical Inquisition factions, such as the Phaenonites.