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Origins:

The Harbringers of Death are were created very late in the Imperium's history, long after the Horus Heresy and after the events in Dawn of War (if anyone considers that canon). During Dark Crusade, the Harbringers of Death were created as a Chapter which inherited the worst from most chapters. Its commander, Marcus Yavin, was a low ranking Space Marine of the Blood Ravens (who was serving on Dark Crusade when he was reallocated into the Harbringers of Death). The bulk of the force was contributed by the Ultramarines. Most of the Ultramarine recruits who were deemed heretical by the High Inquisitor were allowed one chance to redeem themselves by joining the Harbringers of Death. As such, the bulk of the Space Marine forces are of low grade. However, they do have a few elite Space Marines, namely Tactician Andrew Dark and former Black Templar Malak Morad, who train troops and provide battle plans. A cut of the Imperial Guard who come from the 412 Cadian stands as an auxillary force which often invades planets on campaigns before the Space Marines even touch down. Their fleet is tiny, consisting of only one main force, and much of that force is borrowed or salvaged from other Chapters

Fleet and Ship Origin and Description:

Main Fleet: "Vengeance"

Thunderbolt Class Cruiser: "Slicer" (originally an Ultramarine Cruiser, renamed and re-outfitted)
Emperor Class Battleship: "Origin" (created from scratch specifically for the Harbringers of Death)
Thunderbolt Class Cruiser: "Dark Reign" (originally a Space Wolf Cruiser, renamed but not re-oufitted)
Super Heavy Fuel Transport: "Invincible Shield" (salvaged from a crashed transport on Kronus during the Dark Crusade Campaign; re-armored, repaired, re-outfitted for combat)
Dauntless Class Cruiser: "Bravery Barge" (salvaged from a crashed cruiser on Caldaris; re-armored and repaired for combat; still contains original torpedo weapons)

Individual Ship Statistics:

Thunderbolt Class Cruiser "Slicer"
-Fighter Complement: N/A
-Bomber Complement: N/A
-Gunship Complement: N/A
-Drop Pod Complement: 19 Deathwind Drop Pods; 20 bare (no armnament) Drop Pods
-Crew Complement: 5,140
-Imperial Guard Complement: 1,300
-Space Marine Complement: 500
-Docking Facilities: Drop Pod Facilities; 2 Emergency Docking Bays (not to be used under normal circumstances)
-Armnament: 18 Twin Bolter Turrets; 2 Lance Turrets; 4 Twin Torpedo Launchers

Emperor Class Battleship "Origin"
-Fighter Complement: 30 Thunderbolts
-Bomber Complement: 15 Starhawk Bombers
-Gunship Complement: 10 Valkyries
-Drop Pod Complements: N/A
-Crew Complement: 10,150
-Imperial Guard Complement: 4,670
-Space Marine Complement: 1,000
-Docking Facilities: 5 Strike Craft Docking Bays; 2 Transport/Cruiser Docking Bays
-Armnament: 75 Twin Bolter Turrets; 5 Lance Turrets; 10 Twin Torpedo Launchers; 15,000 Stationary Bolters (dispersed throughout the hallways and rooms of the ship; manned by the nearest onlooking crewemember or soldier during an attack)

Thunderbolt Class Cruiser "Dark Rein"
-Fighter Complement: 25 Thunderbolts
-Bomber Complement: N/A
-Gunship Complement: 2 Valkyries
-Drop Pod Complement: 25 Deathwind Drop Pods; 40 bare drop pods
-Crew Complement: 6,550
-Imperial Guard Complement: 3,780
-Space Marine Complement: 1,140
-Docking Facilities: 3 Strike Craft Docking Bays; 2 Emergency Docking Bays; Drop Pod Facilities
-Armnament: 25 Twin Bolter Turrets; 2 Lance Turrets; 6 Twin Torpedo Launchers

Super Heavy Fuel Transport "Invincible Shield"
-Fighter Complement: N/A
-Bomber Complement: N/A
-Gunship Complement: N/A
-Drop Pod Complement: 45 Deathwind Drop Pods; 80 bare drop pods
-Crew Complement: 3,450
-Imperial Guard Complement: 6,150
-Space Marine Complement: 3,950
-Docking Facilites: 2 Drop Pod Facilities; 3 Emergency Docking Bays
-Armnament: 30 Twin Bolter Turrets

Dauntless Class Cruiser "Bravery Barge"
-Fighter Complement: 15 Thunderbolts
-Bomber Complement: 45 Starhawk Bombers
-Gunship Complement: 10 Valkyries
-Drop Pod Complement: N/A
-Crew Complement: 6,170
-Imperial Guard Complement: 7,140
-Space Marine Complement: 3,150
Docking Facilites: 7 Strike Craft Docking Bays; 1 Emergency Docking Bay

Oh... Good point..
Like... Recruits (noobies) of the Ultramarines have to prove themselves, right? (that's what I was taught) If a scout or initiate screws up really bad (and from what I've heard, high inquisitors deem lots of people heretical) then they get executed or the lucky ones end up in the penal unit, Harbringers of Death.

Or...here's always my thought on the matter...

Don't tie them to an existing chapter! Hell, the chapter of CSM that I'm working on are recently turned Imperial Marines, and they came from the tithes of gene-seed that the Mechanicus collects and cultivates. Sure, yeah, they need a parent chapter for some things. Well, their parents are the Brazen Suns, an otherwise nonexistant chapter!

Really, to keep people from jumping at it, just avoid using existing chapters as a parent. Calling them a successor to UMs works if they're a very tight-to-codex force, but not much else.

*Cough*Mortifactors*Cough*

Odds are that there would be rotating shifts of Captains and officers from other Marine Chapters in the area, with a heightened presence of Chaplains to beat the Imperial Creed and Codex Astartes back into their heads.