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 treads 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 originates from). The appearances of these landships as you can imagine, often give anyone from the Adeptus Mechanicus a giant mechanical hard-on due to the rarity of its STC and its humongous size, equivalent to Imperial Cruisers and Battleships.
By the way, this seems to be strongly based on the Mortal Engines quartet, 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 cannibalize each other in a sort of "Municipal Darwinism". A highly recommended series, if you ask me.
History
The history of Zayth can be summed up in one word, WAR. The people of Zayth have been fighting for so long that they literally forgot whatever reasons their ancestors had for starting the war in the first place. The earliest official documentation of the history of Zayth seemed to paint this war torn, ash covered planet as a 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 human colonies due to the Warp doing the equivalent of an acid reflux (blame the Eldar's 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 untrustworthy of one another, and mistrust brought war. It seems the whole "green and verdant" thing didn't last very long after that.
The war grew and continued into lengthy periods of time that Zayth literally forgot what it was they were fighting for. The sheer length time necessary for an entire planet to forget what a global war was about would mean that the war on Zayth must be one of the longest continuous conflicts in the entire Galaxy. PERIOD. The only thing remaining of Zayth's original civilization are the colossal roaming land-ships crawling around the super continent, and the people protected from the now-toxic world outside by said giant land ships.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 particular reason despite well-documented evidence that no one can recall the objective of the war. Occasionally a land-ship falls, and its carcass stripped and looted by the others just like the above-mentioned Cities in Mortal Engines (once again proving GeeDubs is a professional idea thief), 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 are gargantuan crawling Hive Cities on treads, with some ranging from the size of space Cruisers while others (if we take descriptions of the size of some Hive Cities at face value) are big enough to act as docks to full-size battleships. The Landships are armed with powerful city-leveling Macrocannons originating from the Dark Age of Technology, virulent bio-agents (Virus Bombs?), Conversion Beam batteries, and squadrons of missile-laden dust gliders. Zayth forgot why it fights, but it somehow maintained the weapons it fights with intact for literally thousands of years of constant use. Good job?
Each Land Ships are unique, like nations who presumably built them, they preserved (in part) the different cultures and civilizations which effectively haven't existed on Zayth since the Age of Strife. Within these city-sized vehicles exists a rigid caste system, similar to that of the Tau. Presumably this is due to the military or corporate hierarchies, which must have existed for the original crew of the Land Ships, being necessary to keep things ship-shape (heh). The main leaders charged with helming these behemoths are called "Elder-Tacticians", who are essentially Magos' gone tribal. The technology within these obese vessels are a gold mine of STC's, as the technology of Zayth has intact for multiple millennia. That is, assuming it hasn't been replaced with a functional (or sufficiently functional) equivalent or "repaired" into unrecognizable oblivion. This has made the Adeptus Mechanicus a common annoyance for the literal war machines of Zayth, as they tend to love poking things they know absolutely nothing about.