Primordial

Etymology

From Latin primordium, "the beginning, the origin," which fuses primus, "first," with ordiri, "to begin" (a word drawn from weaving, meaning to lay the first threads of a web). So the literal sense is "the first threads" from which everything else is woven. English uses it for whatever belongs to the earliest and most elemental layer of existence.

adjective · /pry-MOR-dee-uhl/
Existing from the very beginning of time, or belonging to the earliest, most fundamental stage of things.
What it names
The word for the deepest, most ancient origins — a primordial ooze, a primordial fear — reaching back to the first state of the world or the psyche.
Example
Standing alone in the vast dark of the cave, he felt a primordial dread that predated all reason.
Related idea
Antediluvian means comically or scornfully old-fashioned, dating it to the ancient past; primordial is grander and more elemental, pointing to the very origin of things rather than merely their antiquity.