Uh, I really don't know if it's the right place to come with this. If it's not, I apologize, please, don't kill me.
So I've recently started a Genestealer Cult army in Warhammer 40k, and I figured that I should create some background for my force to get the hang their fighting style before I start investing absurd amounts of money into it. The problem is, I'm out of ideas. For that reason, I decided to ask the best, the nicest and the most helpful people in the world (that's you). I am aware that most of you are probably unfamiliar with Warhammer 40k, so I'll try to explain the best I can what a Genestealer Cult is about. Brace yourselves, it may get a bit long.
Wait a sec. What are you even talking about? Oh well. Warhammer 40k is a science fantasy tabletop wargame, in which you control an army on the battlefield. It is set 40000 years into the future in our galaxy, except it's full of space magic, evil gods, bloodthirsty maniacs, war machines the size of a small country and aliens suspiciously similar to classic fantasy races (except dwarves). Also, a corpse in a chair is a god of humanity. It's notoriously dark and expensive to play. I hope this short description will clear things up for you if you've never heard of Warhammer 40k at all.
Alright. What is a Genestealer Cult? Simply put, it's a horde of alien-worshipping mutants posing as freedom fighters for good publicity. They worship mighty hive fleets of the Tyranids, known to them as Star Children, as gods. The head of the cult is a Magus, a space wizard with mind control powers. They infiltrate the downtrodden society of their home planet, luring people in with promises of equality and revolution against the tyrants, to prepare for the coming of their alien masters. They fight using mostly repurposed industrial tools and vehicles but have access to some military grade equipment too. A cult is founded when a Genestealer (think xenomorph with brainwashing powers) lands on a planet and starts infecting people with its genetic material, turning them into its loyal servants. Any children they have bear a physical resemblance to it, which becomes less and less pronounced over the generations. When the cult grows large enough, it exports Genestealers to other planets, starting the process anew.
But Ran, what are those Tyranids you mentioned? Well, if you're familiar with StarCraft, they're basically meaner, hungrier Zerg. If you're not familiar with StarCraft, they're an extra-galactic alien race travelling across the stars in vast swarms known as the hive fleets, attacking planets with hordes of bio-engineered monsters with the intention of consuming them. Every individual organism, by itself no more than a vicious animal, is controlled by an outrageously intelligent Hive Mind binding the entire race together.
Creepy. What do you have so far? My Genestealer Cult is known as the Sons of Jormungandr and hails from a cluster of space stations. They worship the Great Serpent in the Sky, also known as Hive Fleet Jormungandr, which favors subterranean assault as the tactic of choice. They dress mainly in black and yellow, mimicking the colors of the aliens they revere. Originating from a relatively isolated community, they've been able to spread relatively unimpeded and have already expanded to several planets in the sector.
What do they want? Main goals of the cult consist of spreading their influence, keeping their true nature secret and acquiring weapons and equipment to prepare for the great uprising and the coming of the Star Children.
If you have any ideas on how to expand upon that, please, let me know. If you have any questions regarding the source material, I will gladly answer them. And once again, I'm sorry if it's not a good place to ask.