Cadiz system

Four hive worlds make up the Cadiz system. A fifth world, St. Perditius, was destroyed through orbital bombardment and is nothing more than a tomb.

St. Anne: A moon, tide-locked to a vast gas-giant in the outer system. A thick atmosphere and the tidal stresses caused by its parent planet generates and retains enough heat for the surface to be habitable. With little agri-produce and very few remaining natural resources, the industries of St. Anne have fallen silent. Reclamation is now the preeminent industry.

St. Lawrence: This world lies at the very outer edge of the star's habitable zone. Once, it was fertile, but the irrigation and heating systems have failed eons ago, and the world has reverted to its natural state, which is endless cold windswept plains. The planet's minor hives are failing, and more and more people are trying to live as nomads in the outback. Most fail, but there are some that persevere.

St. Grumman: This massive Earth-like world is mostly covered in hot oceans. The most heavily populated and, perversely enough, the one least hospitable to human life. Including, but not limited to: crushing gravity, sweltering heat, extreme humidity, and horrendous storms. The billions of people living here live on the major islands, in underwater cities, or floating arcologies. On the plus side, the planet has plenty of biosphere and plentiful deep-ocean mining.

St. Helios: In the innermost orbit lies St. Helios, a sun-scorched hell-hole that cannot support human life outside domed cities and subterranean habitats. St. Helios has no biosphere beyond the agri-domes and the fungal farms in the deep. The planet is the only place with remaining mineral resources in the system. Every day millions of workers risk their lives to harvest the glass forests. About a billion people eke out a living here, many of the descendants from Calixian refugees.

The Dragonborn

 

The Dragonborn are fallen Astartes whose souls have fused with their spirit stone, rather than return to the Warp to be feasted upon by the True Powers and their daemonic servants. This can only happen if the Astartes are fully attuned to their stone and the stone isn't damaged or destroyed. 

It's basically the same process as Eldar spirit stones, except all Eldar are more psychically attuned than most Astartes, so the transfer process is a little rougher for non-librarians. 

And yes, daemons can take the spirit stones and feast on them :-)

The Dragonborn can then be brought back to life by putting a fresh clone into a suit of armor set with the fallen Marine's spirit stone. After a period - typically a few years, but this can vary quite a bit - the soul in the stone will overwhelm the weak spirit of the clone and take full possession of the body. 

Meanwhile, the gene-seed of the fallen Astartes can be used as normal (in game terms, the new Astartes starts with the legacy-bonus from the fallen).

For some unknown reason, the Dragonborn marines are more likely than other Astartes to develop "draconic" mutations. Perhaps it's a result of the cloning process or the soul-transfer. No one knows for sure, meaning the most popular explanation is: Draco vult/The Dragon wills it.

11th Legion "Granite Guards" & the "Loyal Sons" Successors

The 11th legion was mustered from Terran stock, using gene-seed derived from their Primarch, same as every other legion. Unfortunately, this Draco was something of the odd man out, a prophet living among a people without faith. Then he had a falling out with the Emperor over the Lost Daughter. Rather than greatly expand the 11th for coming conquests, the Legion was disbanded, and what remained of its original Astartes broken up into independent chapter-sized units and sent out to die with honor (very reminiscent of what happened with the Loyalist legions after the heresy). 

In the aftermath of the Horus Heresy, gene-seed from the 11th was entered into general circulation (either by accident, on purpose, or through subterfuge-not even the Gods seem to know which), and an unknown number of successor chapters were created from Draco stock throughout an unknown number of foundings. The Sons of Xandor, who were known to operate in from around M35-possibly earlier-to M42, are is probably such a chapter. After Abaddon's ascension to the Throne of Terra, a number of successor chapters banded together under the moniker 'Loyal Sons'.

Dracropolis (ex-Jupiter) juggernaut

The Dracropolis is the first-and only-ship of the Jovian-class star juggernaut. It dwarfs every Imperial ship ever built (not counting the Phalanx, whose construction predates the Imperium). In fact, it is more of a mobile base than a true battleship, lacking any form of tactical mobility (other than slowly moving towards its target area).

Constructed above Jupiter before the Grand Crusade, the Jupiter (its intended name) was never finished. The Tech-priests of Mars simply lacked the templates and technical abilities needed to complete such a massive undertaking. The project was abandoned, but it taught Mars an important lesson regarding the optimum size of major warships. This in turn led to the Gloriana-class of super dreadnoughts, which became the largest class of warship to be produced in some numbers (the three Abyss-class ships-including the Verbum Finalis-are bigger, but also have some size-related flaws).

During the Great Crusade, much lostech was recovered and the Adeptus Mechanicus's ship-building proficiency improved considerably. This led to the Jovian project being restarted (which in turn sparked the Abyss project). After many delays and endless problems, the ship was made spaceworthy and launched, but shortly before it was due to be commissioned, it was seized by traitor marines of the Granite Guard. The Jupiter was never heard from again-it is believed to have suffered catastrophic warp drive failure during its escape from the Sol system.