What or who is Amaranth

An Amaranth is a person who can achieve the status of God. Furthermore, not just a God but the paramount God, the God whose dream created the world. While Amaranths and the creation of the whole world through a dream are not strictly canon in TES, there have been mentions of their existence. For example, part from "Loveletter From the Fifth Era, The True Purpose of Tamriel" states:

The New Man becomes God, becomes Amaranth, everlasting hypnogogic. Hallucinations become lucid under His eye and therefore, like all parents of their children, the Amaranth cherishes and adores all that is come from Him.

It is stated in other books, like "Progress of Truth" in TES: Morrowind that people have tried achieving "godhead", thus being an Amaranth (which means dreaming the dream that is the world) through different means. For example (from "Progress of Truth"):

Temple doctrine claims their apotheosis was miraculously achieved through questing, virtue, knowledge, testing, and battling with Evil; Temple doctrine claims their divine powers and immortality are ultimately conferred as a communal judgement by the Dunmer ancestors [ including, among others, the Good Daedra, the prophet Veloth, and Saint Nerevar ]. Dissident Priests ask whether Dagoth Ur's powers and the Tribunal powers might ultimately derive from the same source -- Red Mountain. Sources in the Apographa suggest that the Tribunal relied on profanely enchanted tools to achieve godhead, and that those unholy devices were the ones originally created by the ungodly Dwemer sorceror Kagrenac to create the False Construct Numidium.

While several people have achieved apotheosis, there are no documented cases of actual Amaranths / someone becoming Godhead in TES. Still, the idea of an Amaranth is not officially canon.