Mastodon
The Mastodon also known as the Mastodon Heavy Assault Transport is a super heavy transport and one of the largest ones usable by tabletop standards. It was utilized by the SPESS MAHREENS! during the Great Crusade and subsequent Heresy and was reserved for use against only the most heavily fortified of enemy positions. Of course after 10,000 years of doing absolutely nothing of value, the production and usage of the Mastodons have either been lost or kept jealously secret by the Techpriests that still protects them, making them a rare and nearly extinct vehicle...kind of like the real life animal; even in the Great Crusade the Mastodon was used sparingly as deploying them was a pain in the ass in itself due to the bulkiness of its size and practicality when cheaper and faster alternatives such as Rhinos are available, being only deployed during long protracted siege operations where one or more entire Legions had amassed to destroy a foe that no other was capable of engaging. Mastodons are used by both the loyalist Space Marines and Traitor Legions and is one of the largest vehicles usable by the Space Marines short of Titans. How large you say? Well it is over 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 from artillery to even fucking Titans! Aptly named as the Magic School Bus by /tg/ due to its mode of transportation and shape.
Combat Role
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 heavy armored transports of the Imperium, having its interior and exterior clad in armoured 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 protected 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.
Armament
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 think its being sneaky by assaulting the Mastodon on its side or rear. The Mastodon tertiary armaments are Smoke Launchers, a Searchlight , and Armoured Ceramite Plating and can also be equipped with up to four Hunter-Killer Missile Launchers . The Mastadon can be used a command tank, and its Skyreaper Battery can be completely replaced with a command vox relay system.
tl;dr its a Shark Assault Boat on treads.
Tabletop
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 the thing doesn't explode and instead becomes a set of ruins. You were driving around in a fortress.