Apsu (Akkadian Apsû) is the Babylonian god of fresh water — the primordial freshwater abyss, and consort of Tiamat in the Enuma Elish. With Tiamat (saltwater), Apsu produces the first generation of gods from the commingling of their waters. When the younger gods become noisy and disruptive, Apsu plots to destroy them; Ea, god of wisdom, kills Apsu first. His death is what enrages Tiamat into war and sets the entire cosmogonic combat of the epic in motion. Apsu is the lesser of the two primordial waters the Enuma Elish names as preceding everything else — but without his murder, none of what follows happens.
Apsu's consort — the saltwater goddess whose revenge for his death drives the rest of the epic.
The Babylonian creation epic in which Apsu and Tiamat are the first two beings, and Apsu's murder triggers the war.
The broader ancient Near Eastern combat motif the Enuma Elish is the paradigm case of.
Reading Genesis 1 slowly, with the Babylonian background — Apsu, Tiamat, Marduk — intact.