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 ying 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.