Mortis Dreadnought
The Mortis Dreadnought is another super secret and uber rare pattern of Dreadnoughts used and kept by the Dark Angels. In their bid to beat the Mechanicus in 40k's answer to Hoarders, the Dark Angels as expected, never ever use the Mortis Dreadnought outside of their hands.
Overview
The Mortis Pattern, like most Dreadnought variants, only differs from the standard Dreadnought in its armament loadout and the battlefield role it plays. Standard Mars Pattern Dreadnoughts are always equipped with a single long-range weapon arm, such as an Assault Cannon, and a single arm ending in a Dreadnought Close Combat Weapon, yet the Mortis Pattern Dreadnought replaces the close combat weapon arm with a second ranged combat weapon. The Mortis Dreadnought is capable of laying down a tremendous amount of firepower upon a target, and since the Dreadnought's body is such a stable firing platform it can continue moving while firing accurately. Essentially think of the Mortis Dreadnought as the yin to the Furioso Dreadnought's yang.
While there are other chapters who utilize the Mortis Dreadnought, the Dark Angels seem to have the majority of the stock as well as the majority of all the good shit back in the Great Crusade. The Mortis Dreadnought is always equipped with two long-range weapon arms, and these two weapons are always the same. The Hellfire Dreadnought, which is a common Dreadnought variant used by most Astartes Chapters, is the only other Dreadnought variant that is equipped with two long-range weapons, and one of those weapons is always a Missile Launcher. The Mortis Pattern Dreadnought is capable of being armed with two twin-linked Heavy Bolters, two twin-linked Lascannons, two twin-linked Autocannons, or two Missile Launchers. These weapon options are capable of turning this pattern of Dreadnought into a highly effective anti-infantry or anti-armour machine, and unlike other Dreadnought variants, such as the Furioso Dreadnought, the Mortis Dreadnought can remain safely at a distance from his target. The Mortis Dreadnought, like all Dreadnought variants, can also be outfitted with extra armour plating, smoke launchers and a searchlight.
Contemptor-Mortis Dreadnoughts
Back in the day of the good ol' Great Crusade, the need for Contemptor variants of the Mortis Dreadnoughts was in a much more accessible supply. Heck, back then the Dark Angels weren't the only chapter legion to have a monopoly on the good shit, other legions such as the Sons of Horus, Death Guard and Iron Warriors made extensive use of these war machines during the height of the Crusade. Then again, the main users of the Contemptor-Mortis is the Dark Angels, guess old habits do die hard.
Like their smaller cousins, the Contemptor-Mortis Dreadnought was able to target and destroy the foe at close range and was also outfitted with an advanced Helical Targeting Array that allowed it to provide a Space Marine spearhead force with close anti-air support when needed. This is thanked in no part due to the usage of the more advanced Contemptor chassis as a potent carrier of heavy weapons. Ten standard millennia later, there remain several Contemptor-Mortis Dreadnoughts that are still operational and are most likely in the hands of the Dark Angels like their smaller Mortis brethrens.
The standard load-out of the Contemptor-Mortis was two sets of twin-linked Heavy Bolters, along with Smoke Launchers and a Searchlight. The walker's Heavy Bolters could be replaced with several different weapons, including a set of twin-linked Autocannons, a set of twin-linked Lascannons, a set of Multi-Meltas or a set of Kheres Pattern Assault Cannons. The Contemptor-Mortis could also be outfitted with Extra Armour Plating and a carapace-mounted Havoc Missile Launcher. In the late 41st Millennium the Contemptor-Mortis can be equipped with a carapace-mounted Cyclone Missile Launcher instead of the Havoc Missile Launcher.
Tabletop
What happens when you combine the upgraded toughness and speed of a Contemptor and the sheer Dakka that is the Mortis? You get the Contemptor-Mortis Dreadnought of course! The Contemptor-Mortis is seen as a immense upgrade of both the normal Contemptor and Mortis while sharing none of their faults. The Contemptor-Mortis has an increased stat-line and relic wargear of the former (The regular Contemptor), and a double-gun load-out and skyfire/interceptor ability of the latter (The regular Mortis).
But wait there's more! The Contemptor-Mortis ignores everything and can take 2 Kheres pattern assault cannons and a cyclone missile launcher to further turn entire armies into finely minced meat. If you find yourself not in range of the assault cannons, move forward and throw down with a couple of missiles. If you DO have something in assault cannon range, hold still and let the rape flow - your dread will loose a total of 12 S6 AP4 rending shots at 24" on a hapless unit and if you stood still, you can drop 2 more missiles on the poor fuckers. You will tear apart tanks, mow down infantry and blast aircraft out of the sky with this fucking thing.