Spartan Assault Tank
The Spartan Assault Tank is a massive vehicle used by Space Marines. It is the "big brother" of the Land Raider line, having roughly the same shape but built to a massive scale; it is as tall and wide as the Land Raider Proteus while being longer, and larger in all dimensions than the standard "Phobos"-pattern chassis. In that chassis, the Spartan can carry 25 Marines (more than twice as many as most Land Raider patterns, and in fact more than any Imperial vehicle other than the super-heavy Gorgon Armored Assault Transport, CRASSUS ARMORED ASSAULT TRANSPORT, Mastodon and Stormlord) in almost perfect safety before unleashing them on the enemy. That's an emphasis on Marines -- even during the Great Crusade, when Land Raiders were plentiful enough to be used by the Imperial Army, the Spartan was strictly Marines- and Mechanicus-only. Even during the Imperium's golden age, the Spartan was just too expensive for mass production, and in the grim, dark future of the 41st Millennium, the only Spartans around are relics of that era.
Fortunately, Spartans are incredibly long-lasting. They have AV14 all around and five hull points -- if they were any more durable, they would have to be classified as super-heavies (and in fact, two vehicles built on the Spartan chassis, the Cerberus Heavy Tank Destroyer and Typhon Heavy Siege Tank, are super-heavy tanks). Its weaponry is similarly up-scaled from the Land Raider, with a twin heavy bolter (which can be exchanged for a twin heavy flamer instead) a pair of quad lascannons for vehicle destruction that can be swapped out for laser destroyers if it needs to go hunting superheavies, and options for a pintle-mounted storm bolter, twin-linked bolters, heavy bolter, heavy flamer, multi-melta, Hunter-killer Missiles, and a Havoc Missile Launcher. For the times when even AV14 on all sides isn't enough, it can also take armored ceramite to make it melta-proof and a flare shield that causes most frontal attacks to dissipate harmlessly. Needless to say, it's considered a must-have for any self-respecting Horus Heresy legion list.
Funnily enough, the original Land Raider, back in the Rogue Trader days, had an extended hull variant called the "Land Raider Spartan", but Forge World decided that a longer Proteus wasn't impressive enough and scaled the whole thing up, hull, tracks, even the sponson guns were doubled up, and adding the assault ramp nose of the modern Land Raider.
While it shares the name with a certain someone, it has nothing to do with Master Chief. Though either through careful planning or a stroke of luck, the rules for the Spartan can be found on page 117 of the Imperial Armour book it appears in.