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, 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).
Funnily enough, the original Land Raider, back in the Rogue Trader days, was called the "Land Raider Spartan", but Forge World used that vehicle as the basis for the Proteus pattern, and designed the new Spartan with the proportions of the modern Land Raider scaled up to the height and width of the Proteus.
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.