All that follows is my own take on things. Since the Hist, and their deliberate use or disuse feature in all of the Dres questline proposals, I think we need to (re)discuss the Hist. Nobody can really agree on how to handle the Hist and the ususal Lore suspects are no help (although I think MK did toss out a few good ideas of them, they‘re not really thought through nowadays).

I don‘t like the prevailing interpretations where the Hist are one of the two masterminds behind all of the TES setting, with the other one being Anumidium. So, with that said...

Hist Physiology

The Hist are pseudonatural beings. It doesn‘t exactly matter if they are from the Twelve Worlds of Creation that preceeded Nirn, from a previous Kalpa, a previous Amaranth, or from beyond the original Outer Void.

Just as Dagoth Ur is not dead, the Hist are not real. When they entered Nirn, or stayed put where Nirn formed around them, the new reality was incapable of sustaining their original form. Nirn was simply too small and too fast or they were too unwieldy and static. Although there was no higher intellect at work, the Hist were transformed into/rendered as trees.

What does that tell us?

1. The Hist are unmoving<
2. The Hist are branching
3. The Hist are rooted.

The Hist are vast soul repositories, self-contained Towers of their previous reality, containg oceans of sap memories. There is no governing intellect at work, they are simply too big and their real mind moves at molasses.

Since Nirn is not aligned with their reality, in their original form they were bleeding their memories from their pores, mortally wounded simply by being real. Some unlucky(?) not-yet lizards licked the sap and by the sheer weight of the souls they absorped were forced into sapience by the rules of the new reality.

Bleeding off their own soul mass into animals was how the Hist were able of sustaining themselves. Since they were realized as a vast forest, the souls that died in their swampy realm returned to themselves instead of going to the new reality‘s Dreamsleeve.

Thus, by incarnating themselves into sapient beings, the Hist are able to permanently exist in the form of trees. All thinking, all acting that is ascribed to the Hist by Argonian religion is the Hist‘s instincts and vague, distant feelings coloured in their incarnations‘ experience (with three exceptions).

Hist Offspring

First are the Argonians, who consider themselves „People of the Root“ and worship the Hist alternatively as their creators, their ancestors, or as themselves.

The Argonians are the most direct descendants of the Hist, with no other sapience inbetween. Although a cultural tradition, ingesting Hist sap ties young Argonians to their Hist, they receive additional soul mass, instincts, vague, fleeting memories. Argonians who never ingested Hist sap may retain some of the instincts their parents had, but they are no longer Hist-descended and thereby defective in some way, although they might feel less lacking and more unburdened (liberated?) by the lack of additional soul mass. It is possible for them to align them to the Hist later in life, with no complications.

When an Argonian shaman ingests enough tree sap the correct way, they gain access to the memories and knowledge contained within the Hist by the way of visions (until the additional soul mass is bled off and returns to the Hist).

Since they usually conduct this ritual when looking for answers and the Hist are not capable of thinking, they can only find means but not ends. The Kothringi were extinguished by a shaman‘s malice, not a Hist master plan. The Argonians might not be „free“ like the other races are, but they are still masters of their own decisions.

It is not possible for non-Argonians to ingest Hist sap and remain sane. Since other races are fully realized, their souls are whole and the additional soul mass causes an unfocused influx of memories and instincts, leading to temporary dementia and caustic visions until enough soul mass is bled off again. In most cases, this will be the soul mass of the Hist. If it is not, a Hist-aligned „hybrid“ is created. Since this is entirely spiritual, no physical change can be observed, although these „hybrids“ are capable of cross-breeding with Argonians.

Hist saplings are a new development since the Marukhati broke the towers. With the additional influx of Tamrielic Souls into Black Marsh, some remained with the Hist whose ternary fission takes the form of tree saplings, who can grow as trees are wont to do.

Hist History

Unkown – The Hist enter reality. Unable to sustain themselves, they bleed themselves off into nearby fauna, uplifting them into Argonians.

Merethic Era – Topal the Pilot explores the coast of Argonia, meeting some of the natives, both beasts and men. His reports fuel the mistaken believe in some of the Aldmeri circles that the Hist are of the same stock as the moving trees of Valenwood.

First Era – Tribal Beasts and men live in Argonia. The Kothringi and Lilmothiil create culture, the Velothi migrate into the northern valleys. Some of the Nedic tribes settle around the rivers.

First Red Moments – The Hist consciously experience brief moments of lucidity when the Dwemer vanish and the Tribunal rewrites history to allow for their existance as gods. Shocked by what they perceive as hostility, they lash out in panic. Led by an echo of this panic, Beast magicians direct root growth to drown the Arnesian jungle, forcing the Cantemiric Velothi to leave their ancestral holdings and join Dres society.

Middle Dawn – The Marukhati selectives break time, allowing the Hist enough „time“ to think. While Cyrodiil turns into an egg that conquers the stars, a Beast looks at a fractalizing tree-god and asks „Why did you create us?“. When a thousand discordant voices reply somethin akin to „We did what now?“, modern Argonian identity is created.

At the same untime, the Dres (then impoverished nomadic scavengers) approach the unfolding fractals with a proposal of their own. Centuries ago, they approached the Sload for technology to make the plains they were pushed into hospitable, but the designs they traded their souls for lack manpower. Slave raids are required to keep the water pumps and paddys running, but it‘s just enough to scrape by.

The deal is simple: Beasts without much of a will to fight back or flee in return for Dunmer souls and bodies. The Hist, for the first time understanding what they are and intrigued by adding something new to themselves instead of mere eternal recycling, agree. The Beasts are not consulted.

The Dres gain knowledge of slave bracers and the Hist-incarnates are instinctively more passive and withdrawn. By sheer force of the deal, all souls that are released in Black Marsh can now be assimilated by the Hist.

The other First Era – Even though the distinct knowledge of the Hist passes into history, shamanic tradition and oral history picks up the slack and the modern Argonians form out of distinct Beast tribes.

Argonia is formally conquered in 2840 and reorganised by the Reman Empire, but the shamanic tradition remain aloof and untouched.

The Deshaan turns into the bread basket of Morrowind by easy slavery, which enables it to prolongued and unexpected resistance against the encroaching Remans.

The second Era – Life in Argonia goes on. Memories of the The Thrassian Plague enable an Argonian shaman to create and spread the Knahaten Flum, leading to a continent-wide death not seen since the first Era. Entire races go extinct, the Kothringi and Limothiil among them.

The third Era - Life in Black Marsh goes on. Since interprovincial travel is encouraged, especially among the Mages Guild and the Legion, Argonians spread far and wide, and citizens of other provinces settle in Black Marsh. The Dreamsleeve and the Hist repositories exchange souls at a rapid pace.

Third Red Moment – When the Nerevarine banishes the Heart of Lorkhan, time breaks and the Hist experience another, brief, moment of panic. Argonians, driven by the need to protect something they interpret as their home province, migrate back to Black Marsh. Which comes in handy during the Oblivion Crisis and its exagerrated retallings are used for political propaganda by the people now in charge.