Dark Soul Tribe
The tribe originated from a group of druids who were shunned from their previous tribes. They were more concerned with dominating nature than taking care of it - a mindset that wasn't universally welcomed. In the harsh Sa'ar Desert, they did manage to find unknown power amongst the stones. It is said that they found an ancient shrine with connections to other planes and this allowed them to draw on multiple versions of nature. The sudden influx of power attracted those willing to form pacts to partake in the raw power and thus warlocks became common amongst the members. There are also several clerics dedicated to deities who represent similar agendas, especially Asmodeus but also Shar.
While most members are considered equal, there is an inner council of four - including the High Priestess. Major decisions are made by this council, the High Priestess breaking ties. Any direct contact with otherworldly beings are supposed to go through her as well, but in reality, the wording of the rules are vague enough that the others can get away with a lot. More powerful, usually senior, members have unofficial power over those below them. Defeating a superior is a great way to gain respect.
The tribe keeps mostly to themselves and will only get involved with other groups if they can see direct benefits. When tey need supplies they can't get from the area they inhabit, they tend to get it from other places where they don't need to deal with as big repercussions. What those living in the desert know about the tribe is very little. The Windswept Tribe have some encounters with them and so do the Fah'Rah, but few others. This has led to them becoming more of a scary story than anything tangible. Children might be told that if they don't behave the Dark Soul Tribe might come and steal them away to unnamed horrible planes.
Besides the abilities granted to the tribes through whatever the shrine is linked to, they are also very well versed in magic theory. It is very common for them to be extremely competent casters. Despite this, there are no wizards amongst them as book studies aren't very valued. Why spend years hunched over books when you could form a pact and know all of it instantly and be able to do other things with your life.
Mythology & Lore
The Dark Soul is representative of the darkness worshipped at their shrine. Even the majority of the tribe's members know precisely what that means, but the Soul can grant immense power. And, in the end, isn't that what matters? The convergence of portals at the location is seen as a sign of the entity's power and represents the many paths to personal strength.
Tenets of Faith
The individual's power level is always the most important to members of the Dark Soul Tribe. They stick together because it is the best way forward for all of them. It's the easiest way to draw on the power of the Dark Soul and seems to please it. Thus, after personal strength, the second most important principle is to bring more power to the Tribe. Anything threatening the tribe is a threat to every member - greater so than a threat to them as individuals. While everyone will want to save their own skin, they will come together to defend the group at all costs if needed.