The Adeptus Ministorum, knowing that the manufactorum complex, and the large data stacks from the Dark Age of Technology that were found deep within, would be of great worth to the Adeptus Mechanicus, planned to use the manufactorum's location as a bargaining chip. After many years of negotiation with the Mechanicus, an agreement was struck, known as the Fornosian Accords.
The Fornosian Accords state that the Adeptus Mechanicus would be allowed to have access to the manufactorum and its precious databanks of ancient knowledge in exchange for providing the Ministorum and any of its agents an exclusive claim to any new technologies rediscovered.
After the Mechanicus combed through the databanks, the only new design to be found were partial fragments for a flame-thrower tank. The designs were sent to Mars, where the data fragments were combined with the STC data for the Rhino and the newly christened Immolator flame-thrower tank began to be exclusively produced for the Ministorum.
It is unknown if the original data for the Immolator required the use of a Rhino chassis or another, unknown pattern of tank chassis, but due to the data being incomplete the true form of the Immolator may never be known.
The Fornosian Accords still stand today, and the design for the Immolator tank are jealously guarded by the Ministorum. It is unknown if the Ecclesiarchy allows the Ordo Hereticus and those taken into its service to build and use Immolators, as is the case with the Exorcist Missile Tank, or if it can only be used by the Sisters of Battle.