Since there isn’t much written on the Marguul tribes of the Seawall Mountains, what do you see as unique to the Marguul that differentiates them from other goblinoid cultures?
What’s been established is that the Marguul are a league of bugbear tribes whose ancestors “rose up against their hobgoblin oppressors to seize control of their own destiny.” While they’re fewer in number than the Ghaal’dar, “For centuries, the bugbears have raided the larger hobgoblin communities… slaughtering the arrogant hobgoblins.” While some have come down to take part in the Darguul experiment, most remain in the Seawall Mountains, “where they continue to battle any creatures that cross their path.” As noted above, they place great value on martial skill and guerilla warfare, and worship the Mockery and the Lhesh Shaarat (“King of Swords”), an interpretation of Dol Dorn.
The Marguul are supposed to be a challenge that makes it dangerous for adventurers and others to travel in the Seawall Mountains. But this isn’t because they’re somehow innately evil: it’s because they have valid reasons to despise and fight outsiders. The people of the Five Nations are the chaat’oor who drove their ancestors into the hinterlands and who built their cities on the bones of the great Dar fortresses. The catch is that they then also hate the Ghaal’dar, who sought to dominate the displaced dar and to rule those hinterlands. While their records don’t go back that far, Marguul anger can be traced all the way back to Dhakaan, where as noted above, bugbears were effectively indoctrinated to support the ruling hobgoblins and to serve as laborers and front-line troops. The Marguul essentially say we are the strongest of our kind. For countless generations you have feared us and sought to control us. Well, you SHOULD fear us and you will never control us again. So again, the Marguul are supposed to be dangerous. But there is reason for their anger, and if their grievances and their traditions are understood, they could become friends or allies. But their initial stance is aggressive, because they believe that only bugbears have ever done what’s best for the bugbears.
Going more into unique aspects of the bugbears, this is where I’d bring in the traditional bugbear affinity for stealth mentioned earlier. The Marguul have always been outnumbered by the Ghaal’dar, and they embrace guerilla warfare. It is in this aspect that they revere the Mockery—what this article calls The Lord of Victory. Just because you’re strong doesn’t mean you have to be reckless. When your enemies have greater numbers and resources, the Mockery will show you the path to victory, even if it leads you through the shadows. Bugbears have a natural gift for stealth, and the Marguul have refined this to an art. I see the Marguul as exceptional hunters and stalkers, viewing the greatest warrior as the one whose enemy never sees them. I imagine them as having a long tradition of stealth duels, with six Marguul entering a hunting ground from different points and stalking and counter-stalking… though to be clear, in such Marguul duels victory would come from striking an enemy firmly with a blunted weapon; the Marguul wouldn’t carelessly spill their own blood. I could absolutely see an adventure in which explorers venture into Marguul territory playing out like Predator, with unseen Marguul hunters stalking their prey, laying traps and slowly weakening them before a final conflict. This is also where the challenge of diplomacy comes in. Again, the Marguul have reason to distrust and hate the people of the outer world; and when you enter their territory, the first time you see one of them may be when they strike to kill.
I can easily imagine a Marguul player character; the ECS notes that a few of the Marguul have descended into Darguun and become mercenaries, and this could lead them out into the wider world. I’d just emphasize that they come from a culture that sees the rest of the world as oppressors and betrayers; they are always suspicious, always ready for danger, and surprised by altruism and kindness. What others might see as dishonorable strategies, they see as smart tactics; look back to this article at Redblade Rrac’s story of the two wolves.