Cerberus Heavy Tank Destroyer
The Cerberus Heavy Tank Destroyer is a super-heavy tank used by the Space Marine Legions during the Great Crusade and Horus Heresy. It is built with the chassis of the Spartan Assault Tank, but instead of twenty-five Marines, it carries a twin-linked Neutron Laser Projector (the same weapon as the Valdor Tank Hunter) and the attendant arc reactor and radiation shielding -- in fact, the extra structure makes it even tougher than the Spartan.
Apparently, the Mechanicus went to a lot of trouble to get the plans for the neutron laser -- they had to recover wreckage "at great cost" from a battlefield dating to the Dark Age of Technology in the Hyades system (a star cluster well-known for its appearance in The King In Yellow, by the way) and reverse-engineer the neutron laser from that. Still, it did provide an alternative to the bulkier turbo-laser weapons, with the "bonus" of risking backlash to the vehicle if the beam bounces off the target.
Miraculously enough, the Neutron Laser Projector is not a lost technology, given Valdor tanks can still be produced in a scant few Forge Worlds like Cyraxus II. The knowledge to manufacture the Spartan chassis though? It's kaput. Naturally, the few tanks who survived the Horus Heresy are treasured relics of a Chapter's armoury. The Exorcists for example, had not fielded their Cerberus for more than a millennia before the Ashen War, and have yet to awaken it ever since.
The Cerberus model was created by Forge World as part of their Horus Heresy line, specifically as part of the release of The Horus Heresy Book One - Betrayal. It later received rules for use in regular Warhammer 40,000 games in Imperial Armour Volume 2 Second Edition - War Machines of the Adeptus Astartes.