Chaldean mythology, generalized term used to refer to all the mythologies of ancient Sumer, Akkad, Assyria and Babylon (Mesopotamian religions)

Mesopotamian religion refers to the religious beliefs and practices followed by the Sumerian and Akkadian (Assyrian/Babylonian) peoples living in Mesopotamia (around the area of modern Iraq) that dominated the region for a period of 4200 years from the fourth millennium BC to approximately the 3rd century AD

As you may already have noticed, no alphabet letter was assigned to the number 9. The reason why the number 9 was omitted under Chaldean Numerology system was because the Chaldeans felt the 9 was holy, sacred, and thus to be held apart from the rest.