Duskfall refers to the slow decline of the old Saxhleel civilization. Little is known about this time period or what provoked this change, though it is clear that the Elder Argonians still communed with and had Hist Trees of their own. Their bows were often made from the bark of Hists, and the Argonians of old were often able to commune directly with the Hist. 
Days of Stone Nests
Pre-Duskfall, its theorized ancient Argonian society was more advanced and unified. Another possibility is, that the ancient Argonians' trade connections simply reached farther than what was believed. The ancient Argonians were adept at metalworking and often used volcanic glass in their weaponry. Some of their work was even possibly inspired by Dwemer designs.
Ancient Argonians viewed Sithis as a destroyer and feared its destructive force. Thus, among other things, they built vast stone pyramids, known as xanmeers, to stand against the destruction. Fearing Sithis to this extent could explain why the old priests of Sithis were able to retain a power position in Argonian society and reign as kings in the pre-Duskfall period. Sithis had a more central role in this old society, and the ceremonies dedicated to Sithis were also different, including blood sacrifices. Not surprisingly, there was resistance to Duskfall among the Sithis priesthood, some of them taking to extreme measures. One priestesses of Sithis named Shuxaltsei managed to return briefly in 2E 583 and attempted to reclaim her temple in Murkmire..
At least one Hist Tree sought to keep their tribe safe from the coming changes, and made a pact with Molag Bal to wait out Duskfall within Coldharbour. This deal backfired, however, as the Hist Tree was soon enslaved by the Daedra and regularly harvested for its sap. This tribe assisted in the invasion of Coldharbour in 2E 582, though whether or not the Tree had finally been allowed to die is unknown. 
Tsofeer cavern, Tomb of Many Spears, Xinchei-Konu monument and the Teeth of Sithis are examples of buildings from pre-Duskfall era. Xinchei-Konu was an ancient calendar that shows how the Argonians viewed time and tried to fight against the change, for example by manipulating weather. The Teeth of Sithis on the other hand is an old temple of Sithis and possibly the largest known.
Duskfall and its consequences
As for what exactly triggered Duskfall is unclear. Sources largely describe it poetically, such as "The Hist led the way" and "the Hist revealed the true path", implying the Hist is attributed to the chance. Immediately following Duskfall, the Hists ceased all communication with the Argonian people for an indeterminate amount of time. It wasn't until the Argonian Jaraleet rediscovered a method of communication that their relationship finally resumed.  As Hist Trees contain within them the collective knowledge of the Saxhleel people, this sudden silence could have been a major factor in the downfall of Argonian society. 
As a result of Duskfall, Argonian culture went through a paradigm shift. Having previously viewing Sithis as a destructive force and living in fear of it, Argonians now started to recognize Sithis not only as destruction, but also change. Simultaneously, Argonians adapted a more cyclical view of time. They believe that the fear of death and impermanence, which they call Shunatei, is the reason both the ancient Argonians and the other civilizations insist on building giant stone cities. Modern Argonians believe that by building impermanent structures, they conquer Shunatei and become one with Change, or Sithis. 
Several relics from pre-Duskfall or Duskfall can be found in Black Marsh, some of them dangerous. One pair of such relics is the Fangs of Sithis, which could be used for mass destruction, turning crops into ash and creating undead horrors from the living. However, using the Fangs of Sithis is said to consume one's soul, driving the wielder mad. In 2E 582, the Fangs of Sithis were discovered deep inside the Sunscale Strand ruins in Shadowfen, Black Marsh.
The Cyrodilic Collections society aims to research and recover such relics and return them to Argonians. In 2E 583, a former member of the society travelled to the island of Norg-Tzel to pursue another of such relics – the Golden Skull of Beela Kaar, an ancient Argonian sorcerer who practiced dark arts. The relic was said to contain the power of Beela-Kaar himself, vibrating with arcane energy and carrying a terrible curse.
- Concordia Mercius' dialogue in ESO
- Crafting Motif 70: Elder Argonian
- The Lost Communion
- Nisswo Xode's dialogue in ESO
- She Who Eats the Light quest in ESO
- How We Came to Coldharbour
- Wisdom of the Ages quest in Coldharbour
- Jekka-Wass Vozei's dialogue
- Nisswo Uaxal's dialogue in ESO
- The Dream of the Hist quest in |ESO
- Xukas' dialogue in ESO
- Looks-Under-Rocks' dialogue in ESO
- The Fangs of Sithis quest in ESO
- Jee-Lar's dialogue in ESO