Malcador Heavy Tank
The Malcador Heavy Tank is an ancient pattern of heavy tank, dating to the Age of Strife (presumably it had some other designation before Malcador the Sigillite came around and they re-named it for him). Like the Macharius Heavy Tank, it resembles an oversized Leman Russ Battle Tank, but while the Macharius resembles a Russ that has been cut and extended in the middle, the Malcador looks like someone at Forge World made a Russ out of silly putty and stretched it.
The Malcador was mass-produced for the Imperial Army during the Great Crusade, where it distinguished itself from other tanks by its speed. Some of the Legions also took it into service to support their Fellblades and Typhons. Unfortunately, its drive was always temperamental at best, and between the loss of knowledge that occurred during the Horus Heresy and ten thousand years of repair and attempted retrofitting, the Malcador of the 41st Millennium is no faster than any other tank and prone to engine malfunction, represented in-game as a fifty-fifty chance of being completely immobilized rather than slowed down by engine damage. In the grim, dark future, a static tank is a dead tank, and so it has since been superseded by the more reliable and cheaper Leman Russ Battle Tank.
That said, the tanks were never destroyed, merely stored (or "moth-balled", as the practice is also known in the real world), and even though their design is out-of-date and ill-understood, they are still made by M'khand and other Forge Worlds in limited quantities to fill the gap between demand for super-heavy tanks and the production of "proper" super-heavies like the Baneblade. Many armory worlds still have large stocks of Malcador tanks on hand in case they are needed for an urgent engagement, and they are still used by second-line forces such as garrison regiments and Planetary Defence Forces. This sometimes bites the Imperium in the rear; during the Siege of Vraks, Malcadors were withdrawn from storage and pressed into battle by the traitor forces.
The utility of Malcador tanks in the fluff (as they were used on Vraks) and questionably on the tabletop is as semi-mobile bunkers. Roll them into your fortified gun line, let their forward mounted weapon savage the enemy, and with Lady Luck and Admiral Awesome on your side, your Malcadors and any supporting forces will peel back to another fortified position before the enemy can dish out too much retaliation. In all honesty, though, the Malcador on the table top would need a notable buff in durability for this to be practical. *cough*Jeremy Vetock, are you listening?*cough*
Like every other tank operated by the Imperium of Man, the Malcador comes in many flavors:
Malcador Heavy Tank
The original stretched Russ, except it has weaker front and side armor than the Russ and its turret can't spin all the way around. It used to be quite good during the Horus Heresy, as its advanced engine made it Fast, and it could mount a Demolisher Cannon and fire its main cannon at full BS and make a Flat Out move. By the time of the 41st Millennium, however, the Adeptus Mechanicus have lost all knowledge of how the engine works so they get extra finickiness while moving the same speed as every other tank. The only advantages that this tank has are that its rear armor is slightly better and that it can soak a few more hits since it's a heavy tank.
In game terms, it really does deserve to have its front and side armour buffed to be on par with the Leman Russ Battle Tank. Its only redeeming quality is that its more survivable thanks to being a super heavy vehicle, though all of its positives end there. Its sponson weapons angles are dreadful, its main gun has limited traverse, it actually has worse armour values than the Leman Russ (as said before), its overpriced for what it brings, and it has the chance to immobilize itself whenever it moves.
Malcador Annihilator
Like the Predator variant of the same name, the Malcador Annihilator has a set of twin-linked lascannons in its turret (though unlike the Predator, these cannons can only turn 30 degrees to either side), and it also mounts a Demolisher Cannon in its forward hull. This is, literally, a blatant rip of the French Char B1 heavy tank.
Malcador Defender
Recognizing that the mostly-fixed-forward turret was a problem, the Adeptus Mechanicus thought up the Malcador Defender. The turret was given five heavy bolters, each of which covers a roughly 90-degree arc, so the tank can shoot at least one and up to two heavy bolters at anything in any direction (in addition to the sponson weapons--lascannons, heavy stubbers, or autocannons). For anti-armor, they kept the hull-mounted Demolisher Cannon. These tanks were used to horrifying effect to defend breaches in the heretical lines during the Siege of Vraks.
Malcador Infernus
It's an Inferno Cannon taken from a Titan and mounted on a tank. When the tech-priests realized that they had run out of room for fuel tanks, they decided to put them in a trailer towed behind the tank. In retrospect, this was not one of their smarter designs. Though it has a longer range and higher strength than the mini-Inferno Cannon used by the Hellhound, it has the unreliable engine (compared to the Fast Hellhound) and a tendency to explode when catastrophically damaged.
Minotaur Artillery Tank
Some Tech-priest thought it would be a good idea to take a Malcador chassis and mount two Earthshaker cannons on it. The cannons are so large that it had to be mounted backwards in order for the tank to remain balanced and not dig into the dirt. Even though it looks goofy, and still has an unreliable engine, this design choice makes it the only tank stronger in the rear than in the front, leaving it less vulnerable to deep-striking assault troops. That and the fact that it can deliver what amounts to a shot from a twin-linked basilisk point-blank. It probably would have been a better idea to put this configuration on a Macharius Heavy Tank instead, though.
Valdor Tank Hunter
This variant was not named for Malcador, but for Constantin Valdor, Captain-General of the Adeptus Custodes during the Great Crusade and Horus Heresy. The Valdor tank mounts a Neutron Laser that somehow causes an electromagnetic pulse in tanks it shoots (in addition to putting a big hole in them), but also risks getting damaged by the feedback if it misses. Because there wasn't enough room in the tank to put shielding around the reactor that powers the laser, the tank is more prone to catastrophic damage and its crew is at an increased risk of getting cancer. Not that the Imperium cares about tank crews' health.
Gallery
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A Malcador Annihilator. Those are some nasty guns, but only if the enemy is between 11 and 1 o'clock.
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A Malcador Assault Tank.