In Babylonian, Sumerian, and Akkadian myth, Apsu (or Abzu) is the deification of the primordial fresh water who was the lover of Tiamat, the primal goddess of the salt water, and together they created the gods.
"A Mesopotamian god of the deep waters. Apsu existed since the birth of time and is depicted as a freshwater sea. He is the source of knowledge and made the lands fertile. His fresh water mixed with that of Tiamat, the deification of the chaotic salt waters, and from there the main gods were born."
Apsu is one of the two ancient demons inhabiting the ruins beneath Nikamimon, the other being Tiamat. Their presence interfered with the Phantom Society's plans of installing Manitou at the site, so Vice-Minister Nishi hired Naomi to eliminate one of the demons to make room.
During the associated Vision Quest, the player must choose whether Naomi eliminates Tiamat or Apsu. If Apsu is confronted, upon its defeat he curses Naomi and kills her as it dies. The god's power is later absorbed by Manitou, which gives it strong physical attacks but a vulnerability to magical attacks.
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