Zayth

Zaythan Land Ships, big enough to probably hide a strike cruiser and slow enough to make an Ordinatus look fast.

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 where mankind WMDs 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".

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 wartorn, 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 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 the people of Zayth literally forgot what it was they were fighting for. The sheer length of 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 supercontinent, 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

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Trivia

According to the 'Rogue Trader Adventure Campaign: Lure of The Expanse' book, it is written that the Capital of Zayth, Zorlan, survived the Big War through the city's powerful shield dome (void shields?), protecting them against the landships and the world's worsening climate. However, as the war drag on, the Capital's location became lost, "buried beneath the shifting ash and dust of Zayth". The landship clans say that the city and its citizens (gottem) are still alive and kicking till this very day, hidden somewhere in the wasteland. (Since the war on Zayth is one of the longest conflicts in the galaxy, I theorize that Zorlan, with their presumably still-intact STC (Zayth is considered a "young" DAoT colony before their World War), keeps producing new landships for a reason or another, like, say, Conquest, or keeping that pesky Imperium off of Zayth. But most of the time, their landships declare independence and seceded from Zorlan, creating new factions, and prolonging the everlasting war on Zayth (Maybe it's Khorne's doing, maybe Tzeentch's, maybe even both.)

It is also written that there exists an ancient alien city beneath the dried ocean trenches of Zayth, "undisturbed and unvisited for a millennia", chances are they are the Necrons. (There probably are Necron landships on Zayth, it's just that nobody can survive long enough to confirm their existence.)