The Colossus Bombard is an artillery piece used by the Imperial Guard for destroying enemies in fortified positions. Though the Basilisk Artillery Gun has a longer cannon, the Colossus wields the biggest, a great siege cannon that lobs shells the size of a man. Rather than penetrating spikes or fragmenting airbursts, its shells create massive shockwaves that defy armor and cover.
It is different from the other artillery pieces in that it is sometimes built on the chassis of a Leman Russ Battle Tank (despite a Chimera being larger). The cannon is so large that it leaves room for only a driver to sit inside, and has no space for a platform for its firing crew. Its shells are too large to manually load anyway, so it comes with a crane to do the actual loading, while gun crew takes positions around the rear of the vehicle to operate the crane and aim and fire the cannon.
When the cannon does fire, there is too much recoil for the cannon's hydraulics to absorb it all; accordingly, the Colossus is equipped with a recoil spade that digs into the ground and braces the tank, preventing it from rolling back excessively.
The Colossus cannon has the same range as the Basilisk Artillery Gun's Earthshaker (with a somewhat smaller minimum range), but fires with strength 6 rather than 9 (despite the shells being bigger). That said, its shells ignore cover saves, and as AP -2 is enough to ignore most armor and S6 will wound almost all basic infantry on either a 3+ or a 2+, it is a potent anti-blob weapon -- unless it is shooting at units with an invulnerable save or a 2+ armor save, it will probably vaporize whatever it hits without any rolls.
The original Colossus model was produced by Forge World for Imperial Armour Volume Three: The Taros Campaign. The rules were later brought into the main-line Imperial Guard army list with the fifth edition Codex. Funnily enough, although the original Colossus is built on the Leman Russ Battle Tank chassis, it had the same armor values as the Chimera and Chimera-based artillery pieces (unsurprising as little armor would be needed from kilometers away). In the IG codex, the Colossus was depicted as being Chimera-based, while Forge World's Leman Russ-based artillery piece is now known as the "Imperial Bombard" (which is still a bit odd, as GW's Epic-scale Bombards were clearly Chimera-based as well).
6th Edition - The Colossus has officially been removed from the Imperial Guard, apologies, Astra Militarum Codex. From now on to use it you'll have to use the Forgeworld Rules which are in Imperial Armour Volume One, Second Edition.
8th Edition - The Colossus is found in Index Imperial Armour - Forces of the Astra Militarum.
Horus Heresy - The Auxillia Bombard has an appropriate front armor value of 13.