The Mastodon, also known as the Mastodon Heavy Assault Transport, is a super heavy transport utilized by the SPESS MAHREEN! Legions during the Great Crusade and subsequent Heresy against the most heavily fortified of enemy positions, and was one of the largest vehicles usable by the Space Marines. How large you say? Well it was several times the size of a Land Raider and was capable of holding up to forty Space Marines, or twenty Marines and two Dreadnoughts, while protecting them against everything from artillery to fucking Titans! Aptly named as the Magic School Bus by /tg/ due to its mode of transportation and shape. The Salamanders supposedly had the most out of any Legion, as during the One-Five-Four-Six ‘Kharaatan’ Campaign, they brought enough Mastodons to transport the entire Legion force present, about 6000 Legionaries, which would be about 150 of these monsters all in one place.
After the Heresy, the Mastodon faded from Imperial production and usage, due to either having been lost, kept jealously secret by the Techpriests that still protect them, or just joining the Stormbird in the mothball bin after the breaking of the legions. On the Traitor side of things, it must be assumed that by the time of the Siege of Terra the Traitors had already lost most of their Mastodons, as the Emperor's palace had to be breached by the more mundane method of massed Vindicator fire. Although, even during the Great Crusade the Mastodon was only used sparingly, as deploying them was a royal pain in the ass due to the vehicle's bulkiness and impracticality, especially when cheaper and faster alternatives such as Rhinos, Land Raiders, hell, even Spartans, were readily available. As a result, the Mastodon was generally only deployed during long protracted siege operations where one or more entire Legions had amassed to destroy a foe that nothing else was capable of engaging.
The main usage of the Mastodon as mentioned is to act as a long distance heavy transporter that was able to carry a large number of SPESS MEHREENS to a relatively safe area or used to assault enemy fortification without being wrecked by heavy weaponry that can put a Land Raider on life support. Once it has immediate contact with a fortified wall, the Mastodon would then use its forward mounted Siege Melta Array that could create a breach through even the thickest of enemy walls and vaporize any poor unfortunate sod standing in front or behind the wall. Once the Mastodon creates a breach, powerful interior servos fire the assault doors open and the Space Marines or its respective Dreadnoughts would begin to assault its position.
The Mastodon is also one of the most heavily armored transports of the Imperium, having its interior and exterior clad in ceramite plating. The vehicle's upper deck is where the driver and crew are usually stationed and is filled with hatches and portholes for when the commanding officers wants to direct the battle. Furthermore, in case the enemy tries to use chemical or biological weaponry through any suspected holes in the vehicle, the Mastodon can be sealed closed on all sides/doors preventing any leaking. The Mastodon is also equipped with two Void Shield Generators, providing them ample protection from the occasional Titan. The reinforced interior shell and thick exterior armour is so sturdy that even when the vehicle goes kaput it will transform into a nearly indestructible wreck of twisted metal, acting as a small bunker that can protect those vulnerable super soldiers from las or melta fire.
The Mastodon is armed with a forwards-firing Siege Melta Array, which is an array of nine melta-weapons to rape enemy walls and fortifications. The secondary weapons of the Mastodon include a turret-mounted Skyreaper Battery for protection from enemy flyers as well as two sponson-mounted Heavy Flamers for those fast moving critters and two sponson-mounted Lascannons for the occasional tank who thinks it's being sneaky by assaulting the Mastodon on its side or rear. The Mastodon's tertiary armaments are Smoke Launchers and a Searchlight, and it can also be equipped with up to four Hunter-Killer Missile Launchers. The Mastodon can also be used a command tank, and its Skyreaper Battery can be completely replaced with a command vox relay system.
Spartan not enough for you? Have a giant 40-MODEL transporter, with a situational D-weapon to boot. The purpose of the Mastodon is to flood the enemy with fire while deploying a small army of models onto objectives. Said small army can include Dreadnoughts (regular Castaferrum or Contemptor only), which count as ten models each. So a World Eaters' commander can stuff 20 of their nasty Tacs, 5 Red Butchers and a beefy Dread all in the same tank. The Mastodon's main gun is a Siege Melta Array - while only packing 12" of range, it puts out 4 small blasts at str 9, AP 1 and the Stone Burner special rule makes each Pen count as d3 against buildings and fortifications. The beast's other unique weapon is a turret-mounted Skyreaper Battery, a 48" Interceptor autocannon-analogue. Two sponson Heavy Flamers and Lascannons round things off gun-wise; Enhanced Defensive Fire allows these sponsons to Overwatch if the Mastodon is transporting one or more units when charged. So can this thing make it to the enemy lines? You tell me - It has not one but TWO Void Shields! Upgrades include the standard Super-Heavy Command Tank, up to four Hunter-killer missiles and the option to swap the Skyreaper Battery for a Command Vox Relay, allowing the Mastodon's owner to modify their reserve rolls by +-1 while it's on the field, and further dampening Deep Strike Mishaps with a -1 on the table. One final note: if a Mastodon is (somehow!) destroyed, roll a dice at -2, if the result is 1 or less (Because apparently rolling a dice with no modifier and 3 or less wasn't complicated enough for GWs) the thing doesn't explode and instead becomes a set of ruins. You were driving around in a fortress.