The Goddess as Primeval Deity existed before the first beginning, as Chaos- God without form existing in a void and having within Herself the seeds of all things, both spiritual and material, all mixed together without order or definition.
Primeval Deity is androgynous, having both feminine (spirit or yin) and masculine (matter or yang) elements. Sometimes however Primeval Deity is portrayed as feminine, since we think of Primeval Deity in association with the Goddess, or Spirit -indeed as the Higher Self of Goddess
For endless aeons Primeval Deity existed in this amorphous and undifferentiated state, in the darkness before the first creation. At first She slept, aware of nothing, resting and recharging, gathering Her energies and merely being -alone and complete in Herself.
But over the course of time, Primeval Deity began to stir; to become aware. First She became aware of Her own existence. In time She began to think, to dream, to question -to desire. She wondered; What would it be like to feel? What would it be like to move? To take action? To be able to receive sensation? To see? To taste? To hear?
And so for aeons Primeval Deity thought, and considered, and wondered. And then;
“Out of Herself, the first Darkness, She divided Herself. Into Darkness and Light She was divided. The God, Her brother and son, Herself and Her other Self, was the Light.”
Primeval Deity divided Herself -One became Two.
All of the physical, tangible, volatile parts of Primeval Deity went into the God, who was the Light. The God exploded into existence out of Primeval Deity in a shower of spark and flame which we remember as the Big Bang, the starting point of physical creation.
All of the spiritual, ethereal, eternal parts of Primeval Deity remained in the Goddess. In this sense Goddess is thought of as a continuation of Primeval Deity, since the consciousness of Primeval Deity is one of the Spiritual parts that remained with Her.
And the God spread out in all directions through the primordial void, a blaze of light and superheated matter. In time His fire would cool into stars, and from stars to Planets. As this happens the vibration of the matter slows, it becomes more dense, seemingly solid.
This slowing cooling, and solidifying, is described as “Falling into Matter.” Symbolically the God is described as having fallen into Seven Planes of existence. These Seven Planes are, from the lowest up; Physical, Emotional, Mental, Astral, Soular or Egoic, Monadic, and Divine.
Seeing the beauty of the God -that is, of the physical Universe- the Goddess was entranced with desire, and wanted it back. She had dreamed of seeing and tasting and doing, but being separate from the God She could do none of this. She could only observe from outside that which She had created. The Goddess wanted the experiences She had dreamed of, and so She desired to take the created Universe back into Herself... But it wasn’t that easy.
The Goddess consulted Her own Higher Self and realized that “To rise You must fall. To become the greatest of all Goddesses, You must first become a mortal.”
So the Goddess “fell” -that is to say She descended through the Seven Planes to the Physical. She did this by dividing off parts of Herself -the many souls- which she sent into the Physical, placing them into physical forms. Each soul was to lead many lifetimes, going through many physical forms in the process. Reading this now, you are one of the lifetimes of one of those souls, which are part of the great Goddess. We are Her children, and also children of the God, for our existence is the accomplishment of their union.
Note: There are many other theories explaining Primeval Deity, this explanation is my own personal belief.