Grey Death 
At the dawn of the 37th millennium, tragedy struck the Imperium. Warp Storm Dionys ripped through over a dozen occupied star systems without warning, a psychic tornado of colossal proportions that left Astropaths blind and billions of souls contaminated by the Immaterium. Even the Adeptus Astartes did not escape untouched, for hundreds of Chapters drew recruits from storm-ravaged worlds, and found Dionys had aggravated ancient flaws in their gene-seed, bringing mania and mutation to the fore. Saint Basillius the Elder, who centuries later would be condemned as a false idol, demanded the right to judge these storm-touched Space Marines for spiritual purity. No fewer than thirty Chapters were found wanting, and such was the Saint's influence that these Judged Chapters accepted a suicide mission: an Abyssal Crusade of penitence into the unconquered depths of the Eye of Terror.
Prominent among the Judged were the Iron Drakes. Well-regarded specialists in aerial warfare, the Iron Drakes had freely admitted to the effects of the Warp Storm: newly-emerged faults in the haemastamen and multi-lung left them reliant on specialist rebreather equipment to perform the high-altitude warfare that was their signature. Basillius' judgement shocked the Chapter, who had thought the damage minor and their reputation firm. Some believed they had been condemned merely as an example, to show that none were above reproach, but now the Iron Drakes were trapped: having willingly submitted to judgement, they could not flee the verdict. Matters only worsened when the Abyssal Crusade Fleet was scattered, marooning the Iron Drakes on Anathrax. A planet overrun by sapient grey vegetation and mad fungus of toxic iridescence, the spore-clogged air of Anathrax left the Iron Drakes grounded and low on oxygen. They were soon set upon by the various Death Guard warbands who fought over the daemon world, harried and worn down, exposed flesh and cracked armour left bulging with fungal growths. And there it might have ended, were the Iron Drakes unwilling to adapt. Unwilling to survive.
Over the next standard century, the Iron Drakes carried out their mission. Scouts mapped the shifting jungles of Anathrax, learned to move untouched through its bloated boughs. Techmarines repaired armour with strange resin as tough as ceramite, engineered jump packs that could glide through air thick with acrid pollen. Apothecaries developed new remedies of sap and spore, studied which growths could be left to flourish and which should be trimmed like bonsai. Librarians developed a sense for the strange seasons of the planet, communed in psilocybin dreams with a creeping intelligence they came to call Ygg. Warband after warband was purged or fled, overcome by a foe who seemed ever-more part of the daemonic jungle they had thought to claim.
By the time victory had been achieved, the Iron Drakes were no more. Their blue armour was dulled to a cesious grey, its bright steel heraldry tarnished to black, fungal growths of every lurid hue protruding from every gap and joint. The Chapter had conquered Anathrax, but in so doing had learned the beauty of its strange garden. Chapter Master Toskr at last went alone to the daemon-tree Ygg, nephew to Nurgle and father of all the daemon world's infernal vegetation, and sealed a pact atop its canopy. He returned to his Chapter bearing the grey fruit of Ygg, surrounded by a chorus of the squirming, iridescent Dhogg-Spawn that had been born from his fallen brothers. As the Astartes ate, the jungle writhed with joy, and the Grey Death was born.
Renegade Trait Keepers of Ygg
The Iron Drakes adapted to the foul fungal jungles of Anathrax, and were reborn as the Grey Death. Now they spread the choking beauty and Empyric spores of Ygg across the galaxy, cultivating the grey garden on worlds uncounted.
Units with this trait (and any GREY DEATH BEASTS in the same detachment) count as DAEMON INFANTRY for the purposes of FECULENT GNARLMAW abilities. CHARACTERS with this trait can summon FECULENT GNARLMAWS as though they had the Daemonic Ritual ability. Each time you set up a FECULENT GNARLMAW model within 3" of any units with this trait during deployment, you gain 1 Command Point.
Warlord Trait Into The Woods
The warlords of the Grey Death shepherd the awful vegetation they call home, erupting from the jungle depths like a nightmare in grey.
Your Warlord can perform a Heroic Intervention if the enemy are within 6" (rather than 3") and can move up to D3+3" when doing so. In addition, if your Warlord charges, is charged, or performs a Heroic Intervention, you can choose one FECULENT GNARLMAW within 3" of your Warlord and move that model up to D3+3", so long as it ends the move closer to the nearest enemy model.
Stratagem Choking Spores (1 CP)
A cloud of daemonic pollen and writhing vines strangle the enemies of the Grey Death.
Use this Stratagem when a GREY DEATH unit is attacked in the Fight phase. The unit can receive the benefits of cover against attacks made in that phase.
Artefact Of Chaos Rusted Drakewings
A corroded relic of the Iron Drakes, these claw-tipped wings puncture bark and steel alike, fastening the wearer to their quarry as firmly as a tick.
GREY DEATH JUMP PACK model only. The wearer can add 1 to its hit rolls when attacking models that can FLY. In addition, the first time the wearer hits a VEHICLE or a MONSTER in each Fight phase, the target suffers a mortal wound in addition to any other damage, and the distance it can move on its next Movement phase is halved.
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Summation: All we have for the Grey Death is a broad origin story and a colour scheme; I've filled in the origin story, which involved nudging the Chapter's fall away from "we got Chaos stuffed down our throats until we turned evil" (never hugely inspiring) and into a more insidious "the abyss gazes also" situation. I also tweaked the colour scheme slightly; still grey-on-black, but blue-grey rather than green-grey (because we do not need another green Nurgle warband) and with lurid mushroom growths to act some actual colour.
In mechanical terms, I wanted to represent a warband that used bizarre terrain-control in the form of creeping daemon-vegetation, and also do what the Vigilus Renegades largely didn't: encourage the Grey Death to actually fight as a Nurgle warband.
The
Renegade Trait has three parts.
First, it lets Grey Death models benefit from Feculent Gnarlmaws, a static "terrain" unit in the daemon list. These benefits are hilariously good; if you're within 7" of a Gnarlmaw, you get a +2 cover bonus and can shoot/charge even if you fall back/advance. You become tough as hell and very maneuverable... but only beneath the branches of your daemon trees. Second, it lets Grey Death characters summon more Gnarlmaws. This is important because Gnarlmaws are Fortifications, which means taking even one of them eats a detachment slot. And since they can't move, the only way to get them anywhere outside of your deployment zone is to use the Denizens of the Warp stratagem (you can do that, if you're all-Nurgle, thanks to your Daemons Detachment of Gnarlmaws). Summoning means you can be more flexible with detachments, and lets you spread the Nurgle love across the battlefield, albeit pretty slowly since you can't Ritual-and-move. Third, it gives you CP when you set up Gnarlmaws right next to Grey Death models in deployment. This encourages you to turn your deployment zone into a jungle, and massages the need for a no-CP Fortification Network detachment to take any Gnarlmaws at all. It obviously doesn't work with deep striking or summoning. The
Warlord Trait is a Heroic Intervention range boost. Less reliable than the Black Templar or Farsight versions, which give you a full 6" move, but this one lets you bring Gnarlmaws along. See, the other ability of Gnarlmaws is that they can inflict mortal wounds on non-Nurgle units within 3" – but it's normally pretty simple to stay out of that range, since they can't move. This way your Warlord can drag the Gnarlmaw into range when your opponent starts a fight outside of the killzone, nudging it forward that extra three-or-so inches.
The
Stratagem is the most straightforward one so far – you lose your precious Gnarlmaw cover save in close combat, but now you can spend
CP to keep it. The Grey Death really don't like Catachans or Imperial Fists, though.
Finally, the
Artefact of Chaos seems like a complete departure that only exists to play on the history of the Chapter... but it's actually there to feth up flyers, because they're the units most capable of quickly getting to you in combat, and of backing off outside of your Gnarlmaw radius when need be. Now your jump pack Exalted Champion can latch onto that Winged Hive Tyrant or driveby Stormraven and carve into it with a chainfist. Worth noting that the reduced Movement can really feth with some vehicles; take a Valkyrie down to 7 wounds, and its Movement suddenly becomes 20-15". That's an autocrash next turn.
