The Hist are a mysterious, extradimensional species of sapient trees found inhabiting Black Marsh on the continent of Tamriel, on the planet Nirn. The Hist are worshiped by the Argonians as deities, while the Hist in turn worship Sithis.
Although relatively little research has been done on the Hist by the other local inhabitants, it appears that they are interconnected through a hive mind. However, it is still possible for a Hist to go rogue and "escape itself"; in these instances, the remaining Hist will purge the rogue tree.
The sap of the Hist has hallucinogenic properties for unknown reasons, possibly as a sort of poison against otherwise would-be predators. This causes the consumer to perceive not only their own enemies but also civilians to be hostile to them, leading to chaos and confusion amongst ranks. The Argonians have however developed a tolerance to this sap over the millennia and may ingest it to feel its full effects without these negative reactions. In fact, the Hist are able to communicate telepathically with those who have ingested their sap, and due to their reverence of these entities, the Argonians nurse their hatchlings on the sap all the way through infancy. It is stated that the Argonians have developed a co-dependency with the trees, and this seems to be true.
The Hist originated from another realm, but during the Dawn Era they remained bystanders in a way between the Ehlnofey -- presumably because of their sessile nature. Unfortunately, this war would decimate their realm as it passed through. Those that survived ended up in the area that would become Black Marsh.
First mention of the Hist outside of Black Marsh was riddled with absolute inaccuracies, and could be stated to have done more harm than good, though it left the people entirely ignorant as to the true Hists' existence. In 2E 864, the Pocket Guide to the Empire, First Edition was released: In this book, it was written that "Argonians are rarely seen outside of their homeland, except for a relatively intelligent strain called the Hist." It also went on to call this "Argonian subspecies" to be "repulsive, but peaceful enough to be tolerated among the human kingdoms, and can be found as far as western Hammerfell".