So we know that it’s the Kyrian that bring departed souls to the Shadowlands.
So how does it work when Bwonsamdi transports Troll souls to De Other Side?
Does he have a special bargain with the Kyrian so that he’s the one that delivers Troll souls?
I’m not sure if this is answered in the Night Fae campaign since it’s the one I haven’t done yet.
This wasn’t answered in the Night Fae campaign. My best guess is that Bwonsamdi has a claim on all troll souls, so the Kyrian bring to the Arbiter who sends them to De Other Side unless she decided otherwise as seen with Zul’jin being sent to Revendreth.
If I remember correctly, bwom circumvents the arbiter and the kyrians to grab troll souls. In Shadows Rising it was literally killing him due to the system not working like normal 
This seems to be the case, yes. Accordding to the Grimoire Mueh’zala did the same, albeit much to the trolls detriment because he claimed their souls only to torture them even in the afterlife and finally devour them. This practice made him very powerful but- unsurprisingly- not very popular, so when Bwon took over he made sure that trolls had a good afterlife and were also hidden away from his former boss. And obviously he was able to do that even after the Arbiter broke and all the other souls went straight to the Maw.
As for Zul’jin, we still don’t know why he ended up in Revendreth, but there are also troll souls in Bastion so it seems Bwom does not claim them all. Then again, in the bfa Vol’jin questline he says that he only ever lost one before. It’s all quite confusing.
Maybe avoided loa if they lacked faith or perished after their detity fell? 
While it turned out to our benefit in this circumstance, that strikes me as an overall too dangerous power to be allowed to exist. Just bypass the Arbiter?
…Well, for one thing-the Arbiter is dumb, like all of Shadowlands, and Bwonsamdi circumvented them by being in the story first.
Second off, Loa seem to all have different ways of gaining power-niches, sort of. Jani’s is the hope and fervor of the poor, Rezans is the honor from royalty, and they all gain power from shrines and such. But Bwonsamdis specialty is souls-with every soul he gains, he gets power. Its not clear if only souls who make deals with him, or just all trolls go to him-but regardless, that means he has people who are worshipping and honoring him for eternity. Long story short-its not implausible that Bwon-mon is able to just ‘nope’ the Arbiter. The only reason his existence hasn’t solved every problem is he’s not really on our side-he’s on his own side of collecting souls. He might have some nice motives for that-for example, giving trolls a good afterlife-but still, he’s very much acting on his own accord, as shown when he makes a deal with Rastakahn, and almost seems pleased when he fails at first.
Or when he-you know. Makes a deal with a mother to sacrifice her childs life to extend her own. He is VERY self-serving, and all goals are secondary to getting more souls.
Several entities and cosmic powers can keep souls from going to the Arbiter: Elune can take souls she deems “Worthy” who fall in combat, Odyn similarly takes warrior’s souls and turns them into Val’kyre and Valarjar in the Halls of Valor, Helya can do the same with the minions of Helheim, Loa, Wild gods and the Emerald dream, Void, Fel, etc.
And even if NONE of these are eligible to claim your soul, a soul can just decide NOT to go with the Kyrian soul healer apparently… you had one job Kyrian soul collector #1739673920, ONE JOB…
But that’s their problem… not my department. 
…Oh yeah…the spirit healers are probably kyrians arent they
They have the wings and blue skin and everything.
…That said, we can’t really acknowledge spirit-healers in Lore anyway for fairly obvious reasons.
Oh, for sure. He is definitely not one of those great, unfathomable, lofty deities. He is a player and a trickster and obviously not highly respected by the likes of Titans and Eternals.
He makes deals with everyone and yes, they will always benefit himself before anyone else. You can trust him only so far as your goals and well-being align with his own and even then he will make sure to mock you every step of the way.
For a deity he is surprisingly approachable and human- or not so surprising really when he was a living troll himself once who was elevated to godhood, and under very questionable circumstances.
But yeah. He is good at his job. Like really good. He succeeds when others do not, and he is not heartless.
During the Ardenweald questline you have to rescue some of his followers and he assures you that you don’t have to worry about those who are already dead because he won’t let the Jailer get them. I found this little detail particularly endearing because it shows that despite his nonchalant attitude he really does care.
In Shadows Rising he even shows a certain degree of sympathy for Nathanos who has come to kill him, even while he uses his memories of Stephon’s death against him.
And he and Vol’jin even have something akin to a bromance, as with his father before. He mocks them but they just mock back ( “Are you to busy now to summon me yourself?” ) - and he is not the least bit offended.
See, quirky thing is as a Loa/Wild God that hasn’t been bound to Ardenweald by the Winter Queen, Bwonsamdi is still technically considered on team Life. Ergo, any claim he places on a soul is like an intervention from Life, and the Kyrians are required to allow holds other cosmic forces place on souls to resolve before they can be collected.
I’d kill to have Bwonsamdi appear for Darkspear and Zandalari players when they died in Azeroth
Thats what I love about him-he IS self serving. His ultimate goal is getting souls, and all thigns are seccondary-but he’s not heartless, or an idiot-he kind of acts like gazlowe in that regard. He treats his followers well, seemingly not only because they have to revere him for him to have power, but out of genuine attachment-even possibly a circle of life thing. He’s not cruel, but he’s certainly not kind. If anything, considering he was a normal troll once, its possible he’s trying to gain power so heavily for the purpouse of protecting the souls in his care to begin with. And it worked! This dude can litterally fend off the Jailor, the Arbiter, and seemingly even Muehzala-the guy he WORKED for. Through sheer number of people who worship him. I think the only reason he’s not considered the strongest loa-and why he didnt try to stand up to Rezan(besides for the fact that Rastakahn was going to come to him anyway, and its kinda hinted he KNEW that)-is beacuse he doesn’t use his power to strengthen his followers in battle, but to protect them in death. He’s just a troll trying to save who he can, and someone whos gotten used to having to make sacrifices to the point of often coming off as heartless. Atleast, thats how I see it
Bwonsamdi pre-empts Kyrian traffic on all deaths occurring in Zandalar.
The kyrian are forced to respect deals and bargains and things made between the living and the forces of death. There’s a world quest from the kyrian anima conductor that has you determine the fate of three souls every day. Do they stay or do they go? A dead naaru. Go. They don’t belong here. A dead dog? Stay. A death knight begging to be released? Sorry not their job back to your body you go for now. With the reasoning being no soul destined for the shadowlands can avoid it forever. And that things like naaru akd demons don’t belong there.
Being said: bwonsamdi has made a deal with his worshippers to protect them in death. The Archon will and does respect that deal. She always respects his deals.
Yeah Bwonsamdi is textbook Chaotic Neutral and I love characters like that. They are unpredictable and you are always eager to see what scheme they come up with next. Bwonsamdi is willing to make deals with evil Loa like Hakkar because he knows it is Bwonsamdi that gets the better end of it in the long term. However he has his limits. That being Zovaal. He is willing to stand his ground and defend his followers from the Jailer and his minions. Even if it means the boss mon as well.
I also like the fan theory that Bwomsamdi had romantic relationships with the Winter Queen and even Eyir because of how much they hate his guts. Eyir was going to kill Talanji on the spot for having “bwonsamdi’s mark” if it wasn’t for the PC being there as well. Clearly Bwonsamdi left a few scorned women in his adventures as a ladies man.