So in ESO we finally got to visit Black Marsh, and finally learned what the Argonians are like, more about their physiology, culture and beliefs. I decided to make this post to compile what we've learned so far. I'll probably forget a few things so I'll update this post if I do.
Argonians go into the Hist when they die
When an Argonian dies, their soul travels into the Hist trees, thus they pass on their memories to each new generation. However, it was shown that the memories can be lost in the "river's current", the passage of time, but if the knowledge is vital it will be recovered by the Hist. This is shown when the Hissmir Hist shows the inhabitants how to use Hist Amber to power the city's defenses against a Dominion attack. What is not explained is what happens if an Argonian dies and they're not in proximity to a Hist. Do they still travel to the closest Hist, or do they simply cease to be, their memories lost forever? It's uncertain.
The Argonians are divided into tribes
It isn't specified if a single tribe controls a region, i.e one for Shadowfen, one for Thornmarsh, one for Murkmire, etc or if there are several tribes per zone. This appears to merely be a formality, as none of the tribes are named and the Argonians don't seem to identify themselves per tribe, but as a unified whole. Though of course not all agree on everything, such as allying with the Dunmer in the Pact. Only the tribes of Shadowfen, Thornmarsh and Murkmire are on friendly terms with the Dunmer, though none of the tribes openly oppose the Pact as the House Telvanni does.
Argonians have no formal leaders, though the Tree-Minders are the closest thing
Similar to the Companions and their Harbinger, not a formal leader but everyone defers to them in most cases. Tree-Minders tend the Hist, are always mages, and their spirits are very closely connected to their village/town/city Hist tree. Each village has only one Tree-Minder.
The Hist play a crucial role in Argonian reproduction
When Argonian females lay their eggs, they will not hatch unless the clutch is placed near a Hist tree. The eggs then form a connection to tree, and then the young begin to grow within. It is also specified that Argonian males have "contests" to see which males will be given the right to fertilize the eggs, the losers must wait until the next cycle.
The Argonians don't "worship" anything
This is kind of hard to explain, so I'll quote Vicecanon Heita-Meen when she was asked about the connection between her people and the Hist:
"This is a difficult question to answer in tongue-words, especially since, like most of our people, I do not ‘worship’ entities of a so-called divine nature, and don’t understand the feelings and behavior of those who do. Since neither of us comprehends the inner mind of the other, how are we to find a mutually meaningful analogy? But I will try. We have rituals, of course, for rituals are the mud that holds together the house of society, but we do not ‘pray to’ the Hist, because our thoughts and desires flow together. The Hist is the river, and we Saxhleel are the standing waves where the river flows over rocks. That is clear, is it not?"
Bonus: Sithis seems to favor them
Bit of a spoiler here.
During one of the Shadowfen quests, the Dominion use the connection between the Hist and the eggs to cause all the young within to die. All the eggs begin dying, one by one. The Kepper of the Hatching Pools requests the Vestige save as many eggs as he can and bring them to him. When they do, the Keeper begs Sithis to take his life in exchange for saving the eggs, which have begun to darken. The Keeper screams, drops dead, and the eggs return to normal.
No ritual was performed on the eggs, all the Keeper does is pray to Sithis. Of course it's unknown if this is indeed Sithis, only the Keeper prayed, and something answered.
Well that's it, feel free to discuss or point out anything I missed.