The primal Source is the essence of the universe. It is not another god or immortal. It is unknown what created the Source.
The next section contains a spoiler. Proceed at your own risk.
The balance or flow of the Source is very delicate and, should it be disrupted, it could destroy the universe. That is why a god's powers flow back into the Source after death. All gods' powers come from the Source but only Source gods can manipulate it. Source gods are capable of tapping into and manipulating the power of the primal Source.
When gods die, Chthonians are able to control the flow of energy into the Source to absorb the dead gods' powers or direct them to another Source god.
Sephirii are able to "connect" with the Source, allowing the Source to communicate directly to others through the Sephiroth.
Known Source Gods Edit
The gods marked with an asterisk (*) are also primary gods.