Typhon Heavy Siege Tank
It was developed when Perturabo asked the Adeptus Mechanicus for a siege tank that could keep up with a Space Marine army (the Imperial Army's artillery being slower than molasses), so they took a Spartan Assault Tank and installed a massive Dreadhammer Siege Cannon (and it really does deserve to be bolded -- 2d6 S10 AP-5 D3 will ruin any army's day). For a while, everyone teased Perturabo for reinventing the Vindicator, but then they realized that the Typhon was so heavy that it could fire its cannon on the move, and plow through buildings and fortifications like they weren't even there, and a single shot can utterly ruin even squadrons of tanks (you basically get a railgun blast).
The Typhon Heavy Siege Tank was introduced in Forge World's first Horus Heresy book ("Betrayal"), with the model having been shown off as a prototype at preceding Games Days and finally released in November 2012. Rules for Warhammer 40,000 are found in "Imperial Armour Vol 2 - Second Edition - War Machines of the Adeptus Astartes"