Fimbulwinter bombs
A relatively new re-discovery supposedly built in secret by unknown members of the Holy Ordos and members of the Adeptus Mechanicus, the Fimbulwinter is a device that through currently unknown methods flash-freezes the atmosphere of whatever it hits, provided that there are enough water molecules. The first usage of this bomb was during the end of the Siege of Triplex, where the forge world of Triplex Phall and the ancient Legio Victorum were consumed by frost by the will of Lord Inquisitor F. Kraln, who declared the loyalty of both to be suspect to "a long-proven history of heresy".
The near complete loss of Triplex Phall, the culling of its Titan Legio, and the decimation of its Knight Houses have led to armed conflict on Mars itself. The Mechanicum's functions and faith were shattered by the blow dealt by the favored servants of their spiritual liege, as well as losing the sacred Triplex-Pattern Lasgun, the many many STCs suspected to be hidden in Triplex Phall, and the loyal skitarri legions sent to probe the forge world for said STCs.