Alexis Polux, also known as David Duchovny in space was the Captain of the Imperial Fist 405th Company, master of the Retribution Fleet at Phall during the Horus Heresy and the first Chapter Master of a - no, THE very manly and very much most Spanishest of chapters, after the subsequent fracturing of the legions. A giant of a man, even for a Space Marine, he was known for his cold and extremely logical mind and demeanor. Unlike a lot of the famous marines from the heresy (especially among the Fists), he was never found wanting for glory or recognition. He simply did his job to the T and got some serious shit done throughout the Crusade, the Heresy and the Aftermath. Never once failing in his duty to the Imperium. Making him the living embodiment of everything his Primarch stood for.
Alexis was born on the Planet Inwit, the same icy hellhole that Rogal Dorn wound up on. Both he and his brother Helias were selected as potential candidates to become Space Marines, and were sent off into the wasteland with the other recruits to complete the trials. After saving Alexis from some kind of monster, Helias was left dangling off the edge of a cliff, holding onto a very badly wounded Alexis for dear life. Alexis, due to not being Bruce Wayne, couldn't pull him back up, but flat out refused to let his brother go. Deciding having the both of them die was not the smartest move, Helias released his grip and fell to his death. Alexis survived the trials and was made a fully fledged Space Marine. The events of the trails dramatically changed his outlook forever, believing that Helias, the stronger of the two, should have been the one to survive. Despite proving to be an excellent Astartes and quickly reaching the rank of Captain. He always worked thinking he was never as strong or as qualified for leadership as his fellow Commanders (in particular Sigismund).
This perspective would shape his career with a humility that made him an excellent strategist and leader of his fellow Battle-Brothers. At some point in the Crusade, he came to the attention of Yonnad, one of the Legion's senior captains and its undisputed master of void warfare. He adopted Polux as his protege.
After the then Death Guard captain Nathaniel Garro told Dorn of Horus's swanky new Treachery party (and Dorn subsequently tried to rip Garro a new anus with his super-Chainsword), Dorn dispatched a fleet under the command of Sigismund to the Isstvan system to get the leftover Heresy cake. Sigismund then decided to remain behind on Terra, so command fell to Polux's mentor Yonnad. Yonnad then pulled a Frederick Barbarossa and died rather anti-climatically on the way, so command fell to Polux. Despite initial reluctance, he took up the mantle none-the-less and got to work. The event in question that killed Yonnad, was a very sudden and inexplicable surge of Warp Storms that had pinned the fleet in place. Polux chose to wait them out, ignoring his officers advising to brute-force their way out, suspecting that something about this seemed all too convenient and there was likely to be some kind of trap on the other end. Polux instead drilled the ever-loving shit out of his men in boarding actions while he waited, in an all or nothing gamble to curbstomp the awaiting forces in space before they could react.
Despite a lot of doubt from his underlings, the plan worked incredibly well and resulted in the EPIC Battle of Phall. The Iron Warriors, expecting the Fists to be under Sigismund's command and to do the predictable thing of landing and fortifying their position, were completely unprepared for what came out the other end. Polux was ready for them and what should have been a roflstomp turned out to be anything but. Heavy casualties were inflicted on both sides, but once the initial assault was over, the Fists had absolutely gained the upper hand despite the Iron Warriors' numerical advantage.
During the battle, Captain Amandus Tyr informed Polux that the Iron Warriors' flagship was amongst the enemy fleet. Polux then thought 'What would Horus do?' and ordered Tyr to send a Strike Force right into the heart of the enemy, board the Iron Blood, and shoot Perturabo into oblivion (a fitting end). They came unbelievably close to killing Perty (in his own bedroom no less) but ultimately a Primarch is a Primarch and a royally pissed-off Pert repainted the walls of his chambers with Imperial Fist blood (and brains, and viscera, and armor parts...) This did cause Perty's paranoia to skyrocket which lead to a lot of problems for the Iron Warriors, so there is that at least.
Victory was almost at hand for the Fists, but then Dorn sent a Grindr message ordering everyone left back to Terra to help prepare for the final battle. Understanding that they had done what they needed to do, Polux pulled out and headed home, leaving the Iron Warriors with a hollow victory, a raging lunatic for a leader and not enough resources to make much more of a real impact for the remainder of the Heresy until the very end at the battle for Terra and after Horus' death.
Sadly enough, the signal the Navigators picked up as the Fists retreated wasn't the Astronomican but the Pharos instead. Polux and his pals ended up in Ultramar where Granddaddy Smurf was doin' some shady shit, yo! Polux didn't take it meekly and called Guilliman out on it but he was in the end too loyal an Imperial to go against a Primarch and he dropped the subject, especially when both Lion El'Johnson and Sanguinius seemed to be a-okay with Guilliman's practical solution. Polux stayed well-clear of the political bullshit, limiting himself to offer a couple of pointers about fortifications.
Due to some Warp/Pharos-related shenanigans, Polux eventually ended up on the planet Sotha, where he proceeded to assist Warsmith Barabas Dantioch and his still-loyal Iron Warriors to lay down a beating on the Night Lords. What's that? Hating the Iron Warriors just because they were Iron Warriors? None of that. Both Polux and Barabas Dantioch were gifted with the rarest thing in 40K, namely common sense, and they worked together just fine, even becoming friends despite their Primarchs' hate of one another.
After the Siege of Terra, Roboute Guilliman ordered all the Legions to break up into Chapters. Polux was given command of the newly founded Crimson Fists, comprised of the more 'level headed' Marines of the Legion. While the more zealous Black Templars went off to spank the crying traitors all the way back to the Eye of Terror. Polux and his chapter were sent off to combat a Kabal of Dark Eldar, which would set the trend of the Crimson Fists being VERY good at killing Xenos.
800 years after the heresy (making him fucking old for a Space Marine) Polux met his end at the hands of a poison from a massed Scythian attack. But Polux wasn't one to die without getting the job done. With his final words, he gave to his successor instructions on how to eliminate the threat of the Scythians once and for all. With his last breath, Polux ensured the reduction of a formidable race of Xenos from an extremely dangerous enemy of the Imperium to an obscure and fractured band of Dark Eldar knockoffs.
On The Tabletop
Polux was introduced to the Tabletop in Extermination, and Forge World did a bang up job of representing him. With his rules matching his fluff perfectly and also being a very crunchy choice for any Imperial Fist army to boot!
He's a void-hardened power-armoured Praetor with -1 BS/A (BS5 with legion tactics though and his attacks stat is redundant - see below), but with S5, a master-crafted Power Fist with a gauntlet mounted Combi-melta and a Vigil pattern Storm Shield for 3++. You'd normally need Terminator armor for that, but he's that strong. Also, as a master of void combat he lets you teleport in an Imperial Fist infantry unit of your choice via Deep strike and he allows his unit to pass/fail morale or pinning checks at his leisure. Lastly, if he is the warlord he can redeploy one unit in his army before the turn order is determined, with the option to even put it in or out of reserves. A nifty special rule he has lets you drop the Unwieldy rule from his mighty fist in exchange for dropping his attacks stat down to 1, but you still get 2 attacks on the charge! Who needs a Paragon Blade? Go around knocking characters out in one shot with his master-crafted fist, which rerolls to hit in challenges!
Castor and Pollux, the Dioscuri twins (aka the Gemini) were twins born from Zeus fooling around with some married mortal woman (as usual), disguised as a swan, no less. In several versions of the myth, only Pollux had divine blood, and thus was immortal, while Castor wasn't. Castor promptly dies (not unlike Helias here), and Pollux asks to the gods to share his immortality with his brother, so they turn them into the Gemini constellation. Pollux from 30k managed to turn into a full-fledged marine, even a chapter master (of a manly spanish chapter), so you could say he achieved immortality, while his brother didn't.