HateDread wrote:...I'm getting back into the game after 10 years, where I used to play Necrons but grew tired of the lack of wargear. I've got my eye on Space Marines and the Imperium, but find the idea of playing the "100% Loyalist" chapter stereotype a little bit uninspired (no hate to
SM players

)...
There are three golden rules of "your dudes" fluff to keep in mind here:
1. Don't contradict other peoples' fluff. If your Ultramarines are insurgent revolutionaries committed to tearing down a corrupt system they shouldn't be the Ultramarines Chapter headquartered on Macragge and ruled by Guilliman and Calgar, they should be a Company lost in the Warp and written off as dead by everyone else, or a Successor Chapter that has nothing to do with the Chapter leadership, or otherwise be
your dudes rather than rewriting fluff other people have based their own fluff on. People are entirely happy if you tell them what your army is like but get unhappy when you tell them what their army is like, and this approach lets you preserve creative freedom without telling other people what their own army is.
2. Don't overwrite the metaphysics. Chaos is uncontrollable and absolute, psykers risk daemonic possession, faster-than-light travel requires warp drive, etc. Your dudes are your dudes, you have creative control over them, but they need to fit into the setting alongside everyone else and breaking fundamental metaphysical truths that the setting relies on makes it hard to make them interact with the rest of the universe. If you have non-Warp FTL, for instance, why does nobody else?
3. Don't make your dudes too special. Resist the urge to show up canon characters in your own lore, don't Forrest Gump your way into great events. If everyone in the setting has to be everywhere important all the time you end up with a Star Wars Legends-style everyone-knows-everyone setting where all pilots were at Endor, and all Jedi were lucky survivors of Order 66, and nothing is special or noteworthy anymore because everyone was there. This doesn't mean
don't mention great events, they're a good way to tie your lore into the setting, but if your Chapter has a storied history where they were at Prospero and Istvaan and the Siege of Terra and fought the Beast and sided with Sebastian Thor and were present for every Black Crusade and witnessed the fall of Cadia and the awakening of Guilliman it ceases to be special and interesting that your Chapter was present anywhere at all. Similarly if you take a leaf out of the Space Marine Codex-writers' book and insist that every character has dueled an Eldar Avatar just to show off how special they all are it becomes tedious and uninteresting. The first time a Space Marine dueled an Avatar or a Greater Daemon and won it was dramatic and exciting, the fiftieth time everyone starts asking "...Does the Avatar/Greater Daemon ever win these fights, or do they only exist to get punched out by Space Marines?" and the achievement ceases to be special.
If you keep those three rules in mind you can get away with almost anything.
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Background/Ideas:
Maybe an Inquisitor who's seen too much finally cracks after seeing people being abandoned on Cadia? They hold back a transport to help people evacuate - Cadian Guards and Space Marines included - and now they've banded together to push back after questioning what the point of everything is when they're so easily left behind and feel that the sacrifices regularly made by so many are meaningless. One backstory option of many, but you get the gist - they're not Chaos, and they don't want to go around murdering or pillaging. They just disagree with the Imperium's no-holds-barred style
I'm picturing a group of insurgents trying to do what they think is the "right thing" and push back against an "oppressor", fighting in the same vein as in something like XCOM-2; out-matched and out-gunned at a macro level but doing okay at micro, preferring underhanded tactics, surprise/ambushes, etc. Their gear is rusty and poorly-maintained since they don't have a chapter armoury, their looks don't match due to scavenging parts, and so on. Firefly meets XCOM-2 meets the 41st/42nd millennium.
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Plausible. There are two difficulties here: if you make your Inquisitor too much a mouthpiece for modern humanist sensibilities it becomes difficult to believe they were an Inquisitor (ex. Drizzt is an obnoxious mary-sue in part because it's very difficult to believe he was a drow). You get around this one by setting up a reason for the Inquisitor to make this decision that doesn't rely on a sudden emotional urge to do the "right thing" (this is kind of a deus-ex-machina). Maybe the Inquisitor has had a formative backstory event where they fought alongside PDF troops that impressed them with bravery and loyalty, only to see the troops executed on suspicion of daemonic corruption because they had overly-close contact with daemons. Maybe the Inquisitor has had to explain an Exterminatus to civilians whose planet was being destroyed and found they couldn't. It doesn't matter hugely what it is so long as it appears naturally rather than suddenly. The second difficulty is that if you want to mix and match renegade forces (Space Marines, Guard, Inquisition, etc.) you need to set them all up independently, they shouldn't all just sort of come to the same hive-mind decision and fall into like-mindedness naturally.
One other thing to consider is that there's a world of gradation in between "happily goes along with everything the central government says up to and including massacring civilian populations" and "cackling daemon-worshipping berserker who bathes in the blood of innocents". A Space Marine Chapter, for instance, isn't an easily-replaced asset and if one starts asking questions the Imperium's first collective response isn't "HERESY! BURN
IT!". Look at the Badab War; the Imperium didn't assemble a crusade to bring Huron to heel until a hundred and ninety years
after he'd launched an open rebellion and declared himself Tyrant of Badab. The deviation from doctrine that turned into open rebellion probably began decades or centuries earlier, and the renegades didn't turn into possessed/corrupted/Chaos-worshipping cultists until they were driven into the Maelstrom at the conclusion of the war.
The Imperium cares way more about results than it does about enforcing doctrinal compliance; if you pay your taxes, show no overt signs of Chaos-worship, and fight the enemy when called you can do pretty much whatever you like.