Zayth
Zayth is a Hive World in the Koronus Expense where it is dominated by a single super continent. So what is it in Zayth that makes it so unique? Well...the Hive Cities move. What we mean is imagine a vehicle with threads big enough to run over a entire Titan legion and have the armaments to go toe to toe with the most powerful ships in the Imperium. As you can imagine, the technology needed to create these machines has since been long lost as it was created during the Dark Age of Technology (AKA where all the awesome shit use to originate). The appearances of these landships as you can imagine, often give anyone from the Adeptus Mechanicus a giant mechanical hard on as the fact that the rarity of its STC and its humongous size equivelent to Imperial Cruisers and Battleships.
By the way, this seems to be strongly based in the quartet Mortal Engines, written by Philip Reeve which deals with a similar setting were mankind nukes itself nearly into oblivion and after a few centuries they recovered enough to build steam-punk mobile cities which canibalize each other in a sort of "municipal darwinism", a highly recommended series if you ask.
History
The history of Zayth can be summed up in word, WAR. The people of Zayth have been fighting for so long that they literally lost whatever reason their ancestors fought against. The earliest official documentation of the history of Zayth however, seemed to paint this war torn, ash covered planet as a once green and verdant world not unlike Terra in its infancy. Like most things in the Age of Strife Zayth was cut off from the main colonies due to the Warp doing the equivalent of an acid reflux because of the Eldars whole murder orgy thing.
Due to being cut off and being faced with what would most likely be a resource shortage, the people of Zayth started to grow mistrust, and mistrust came war. The war grew to such lengthy periods of time that they literally forgot as mentioned, on what they were fighting for. The lengthy time for them to forget would mean that the war on Zayth would be one of the longest continuous conflicts in the entire Galaxy. PERIOD. The only thing remaining of Zayth's civilization is the roaming land-ships crawling around the super continent and its people; protected from the now-toxic world by thick adamantium hulls and flickering void shields. Like all things in the Imperium, instead of stopping and asking each other what they originally fought for, these dumbasses continue to fight for no reason now. Occasionally a land-ship falls, its carcass stripped and looted by the others just like the above mentioned Cities of Mortal Engines (once again proving GeeDubs' unoriginality), and Zayth takes another slow and steady step toward the day it will become a dead world.
The Land Ships of Zayth
The ships of Zayth as mentioned are gargantuan crawling Hive Cities on threads, with some ranging from the size of cruisers while others; if we take the size of some Hive Cities literally, are big enough to dock entire battleships. The Landships are armed with powerful city-levelling macrocannons originating from the dark age technology, virulent bio-agents (Virus Bombs?), conversion-beam batteries and squadrons of missile-laden dust gliders.
Each Land Ships are unique, like nations to themselves they present different cultures and civilizations ever since the fall of Zayth back during the Age of Strife. Within these clans exists a rigid caste system, similar to that of the Tau. The main leaders of these behemoths are called the Elder-Tacticians, who are essentially Tech Priests gone tribal and are charged driving these vehicles wherever they go. The technology within these ships are a gold mine for STC's as technology within Zayth, if it has not been replaced due to damages are still kept intact for multiple millenial. This has made the Adeptus Mechanicus a common annoyance as they usually poke things they do not know of, much to the Zaythans dismay.