The Hist are a species of giant spore trees native to the province of Blackmarsh, appearing in the Elder Scrolls series. Most knowledge on the Hist is unknown to the greater parts of Tamriel, as the trees are only found in the innermost swamps, which are completely uninhabitable to all save the Argonians, the lizardmen of Blackmarsh. The Argonians claim that these trees aren't what they appear, but rather a sentient race that has existed since the war between the Ehlnofey, long before the ages of man and mer. These trees also tie into the Argonian's religious beliefs, with those Argonians who either don't drink the Hist sap at birth or are born outside of Blackmarsh being unable to hear the trees and, by extension, are considered to not have a soul.
The Hist as a species are connected at the root, with all Hist trees being part of one singular organism. However, this doesn't stop certain trees from cutting themselves off from the greater network, essentially becoming rogue trees. The best example of this is the city tree of Lilmoth, which separated itself in the Third Era, and then again in the early years of the Fourth Era. The tree would be destroyed after the first time it broke free, with a new tree sprouting from one of the few surviving roots and cutting itself off again. This second Lilmoth would later summon the floating island of Umbriel, an extension of the daedric artifact Umbra, into Nirn. The Hist also shares a link to the Argonians themselves, somewhat similar to the link between the queen and drones of a hive-minded species, particularly fictional hive minds such as the titular Aliens of the Alien franchise and the Rachni of the Mass Effect series. Though the Argonians retain their autonomy and free will, the Hist is able to speak to them directly, no matter where on Nirn they might be, if the Argonians in question have consumed Hist Sap, and can use this connection to summon them to protect itself. This was best illustrated during the Oblivion Crisis. Whilst the rest of Tamriel was struggling against the daedric invasion of Mehrunes Dagon, the Hist, having foreseen the events to come, called all Argonians back to Blackmarsh, where it then sent them through in such numbers and with such ferocity, Dagon's lieutenants were forced to close any and all gates into Blackmarsh.
The Trees of Umbriel[]
Whilst the Hist is native to Tamriel, there exists a species of tree that is considered, if not Hist then a distant "cousin." This Umbrielian Hist is, as it's name suggests, native to the floating island of Umbriel. Like the island itself, they originated in an unknown realm of Oblivion, before being transported to the Fields of Regret, the daedric prince Clavicus Vile's realm of Oblivion, and then from there were cut off by the sword Umbra when it brought the island into Tamriel. These trees were slightly different in appearance compared to the Tamrielic Hist; their leaves were too oblate, their bark less fretted, and they smelt a bit off. Following the destruction of the second ingenium, the trees were responsible for teleporting Umbriel to their original realm of Oblivion, though rumour has it that the Sleeping Tree in Skyrim grew from a spore which fell from a "floating island," suggesting it may be the last Umbrielian tree still in Nirn.