The Faash were a human civilization that emerged from the Dark Age of Technology that came into conflict with the Imperium of Man during the Great Crusade. Their homeworld was Dulan.
Like the contemporary Interex they were technologically advanced on a par with, if not already exceeding, the capabilities of the Adeptus Mechanicus. Also like the Interex, they offer an interesting insight on human life separate from the Imperium after the Age of Strife: where the Interex were Noblebright and sought to include various races of Xenos in their empire, the Faash actually abhorred the Alien, Mutant and the Heretic long before the development of the Imperial Creed and without any knowledge of god-like beings such as the Emperor or the Primarchs. You could say they were... Faash-ist?
The Faash were ruled by their own Emperor, Durath, but unlike the Imperium, this leader was simply a regular, un-enhanced man with a strong personality cult and a belief that he was the only person fit to rule humanity. This "creed", along with the fact that they had cathedrals was an indication that they were at least a quasi-religious empire and their level of belief in their cause meant that they were unimpressed by Adeptus Astartes, not demonstrating any of the awe that human forces typically suffer when faced down by power armoured superhuman giants (quite possibly because their own warriors were even larger than Astartes; more on that below). Durath himself showed no fear of Lion El'Jonson and even went so far as to broadcast images of deranged Wulfen as abominations and mutants and holding them as examples of what the Imperium could tolerate that the Faash would not.
The Dulan Campaign
The campaign against the Faash started due to their refusal to join the Imperium plain and simple; the war initially started against the Imperial Army but the Space Wolves were called in after the technological superiority of the Faash became apparent and so Leman Russ was assigned command over the campaign. However, after months of slow progress, the Imperial forces still had not even determined the location of the Faash homeworld and intelligence on their capabilities and disposition was hard to come by, in part due to the sheer resistance of the Faash vessels making them virtually impossible to capture. Not helping was the fact that the Faash Tyrant Durath continually goaded the Space Wolves, calling them the lapdogs of the Emperor and making the whole thing personal, with Russ swearing that he would see their leader dead by his own hand. At this point, the courts on Terra were beginning to consider Russ to be ill-suited to the campaign and decided to send in his brother Lion El'Jonson to complete the job. Incensed, Leman Russ redoubled his efforts to end the campaign himself and managed to actually a capture a Faash vessel intact, thus gleaning the location of Dulan and sending his fleet there en masse.
Upon arrival in the Dulan system, Russ discovered that the First were already there engaging the fleet in orbit, having determined the location of the homeworld by checking through the old records that survived the Dark Age of Technology. Russ had spent months tearing his foes apart searching for their heart, while the Lion simply read it in a book and went there directly. Upon transiting into orbit, ignoring hails from the Dark Angels fleet, the Space Wolves decided to choose their own targets and started shooting down enemy battleships, with one ship managing to destroy one cleanly, only to come under fire from a Dark Angels ship, because an entire company from that ship had already boarded the battleship and were close to capturing it and turning against the enemy, and had been constantly trying to communicate that fact to the wolves; if the Wolves had actually answered their hails they would have known about this. Realising his error, Russ would later reluctantly agree to apologise to his brother, though as the brothers embraced Russ snarled to the Lion that he would end him if he pulled a stunt like that ever again, which surprised the Lion, as he hadn’t actually been apart of the incident at all. This was the first time that the Primarchs had met and they took an instant dislike to one another; they would also disagree on the exact nature of their mission and by extension the function of the Primarchs themselves: Russ argued that their function was to destroy the Emperor's enemies and leave no trace of them behind, while the Lion argued that their function was to win wars and to conquer worlds and that everything in between was irrelevant, drawing the point back to the fact that the campaign had taken the Space Wolves long enough to complete that the courts on Terra felt that Russ was delaying unnecessarily.
In any case, the two Primarchs joined their forces for the final assault: The Dark Angels would assault the outer palace and create the opening for the Space Wolves to insert and eliminate Durath and end the campaign, allowing Russ the satisfaction of slaying the man who had insulted him. Upon planetfall however, the Space Wolves of the 13th company became aware of some of their MIA battle-brothers who had been taken prisoner had since devolved into their Wulfen state and were having their images being broadcast to the population as abominations and examples of the evils of the Imperium and what they were fighting against. The 13th Company diverted their forces to rescue them, forcing Russ to delay his planned assault on the palace to have the 13th company account for themselves. Meanwhile the Lion had completed his task and was waiting for the Space Wolves to begin their phase of the assault, his forces still fighting to hold maintain the opening created earlier. When the Space Wolves refused to answer hails, the Lion decided that enough was enough and assaulted the palace directly, managing to behead Durath and end the war there and then. When the Primarch of the Space Wolves found that he had been denied his promised kill he was furious and knocked the Lion a square blow; blades were drawn and the two started to fight. It did not take long before the Lion disarmed Russ of his weapon, stopping his blade moments away from delivering a killing blow, satisfied that the issue was finished the Lion went to leave but Russ was having non of that and he struck the Lion again; the two descended into a savage brawl that lasted the rest of the day. After the two had beaten each other bloody the fight finally reached its conclusion with Russ struggling to get back onto his feet and the Lion approaching him upon unsteady legs. Russ, upon failing to rise from his kneeling position, started laughing at the absurdity of the situation; they had started the fight as lords of war and now they looked little better then a pair of bar brawlers. Seeing Russ' sudden laughing fit the Lion asked him if he yielded, which only made Russ laugh even harder; even now after the two had beaten each other to an inch of death the Lion still considered this an honour duel. Lost in his fit of laughter, Russ never saw the scowling Lion land the punch that knocked the Wolf King unconscious.
The whole point of the matter was the culmination of neither brother fully understanding the other's thought processes: the Lion did not realize the importance that Russ placed on killing Durath to maintain his honour, and when Russ went off-plan he continued prosecuting his war in the quickest manner possible in order to prevent any further losses to his legion. From Russ's perspective, by sending the Lion an unprovoked punch he did not realize that the Lion would actually take such an action seriously and that the initial blow required an appropriate answer. Russ only later figured out that the Lion had disposed of the broadcast evidence of the Wulfen without telling him when that evidence could have been used to break the Space Wolves legion if it had been made public on Terra. He eventually came to respect the Lion as one of his closest brothers and mourned him when he found out that he was apparently dead prior to the Second Founding.
The Space Wolves and Dark Angels still emulate the brawl whenever they both meet, each sending a champion to fight the other. The original meaning of the duel may have been lost over the course of time, as historically both Codexes give slightly different accounts of the story even across different editions of the same codex. Even Russ had to explain the story to one of his Blood Claws who had not been made aware of the account by his priests or commanders, with Russ considering the Blood Claw's presence a humorous irony when he had just received word of the Lion's disappearance.
As mentioned previously, the technological capabilities of the Faash were extremely potent when compared to the Imperium. Their voidships were powered by unique fusion drives which outran Imperial ships at straight line speeds, meaning they were very difficult to outmaneuver unless they were caught off-guard. Additionally, their ships were extremely resilient, protected by Aegis Shields which were apparently superior to Void shields and could only be overwhelmed by extremely high energy attacks. Aegis Shields were also capable of some measure of self-repair so that when they became damaged they would spring back to function within a very short space of time, making it virtually impossible to defeat a Faash vessel by any means short of destroying it outright. Not only that, but the Aegis Shields could be miniaturized and worn by its Mech-Guard infantry, making them proportionally resilient and difficult for the Imperium to deal with.
The Infantry soldiers were equipped in their own version of Power Armour, built on a scale slightly bulkier than that of the Astartes; these mech-guard were armed with Interference weapons that rearranged matter at the molecular level, causing horrific injuries no matter what protections its targets were using, though these interference weapons took a long time to recharge between volleys. As a backup they were also armed with standard las weaponry that they could switch to in the interim. With all of their protections and the potency of their armament, the warriors of the Astartes soon determined that engaging the Mech-Guard in ranged combat was not effective as it would take a ridiculous amount to firepower to bring down just one opponent; they learned to wait until their interference volleys had completed before closing to engage them in melee where both sides were more evenly matched in terms of brute strength thanks to the power armour, but the Astartes had the advantage due to faster reactions and more savage instincts.
The Faash also had their own version of a Dreadnought called a Leviathan which incorporated the same Aegis Shield and interference weapon as the infantry, albeit on a larger scale. These Faash Leviathans were actually even larger than Imperial Leviathan Dreadnoughts and were a horrifying prospect to take down.
As usual, despite conquering several worlds that were superior in some fields of technology, the Imperium is never depicted either before or after the Great Crusade ever using said technology. Likely because Imperial las weaponry is used for its ease of extreme mass production and Imperial top end tech (volkite, conversion, graviton, etc) is far superior.