Nemed was the original God of Life. He gave up his precept to Arawn (who was before then only god of Death, as he would again be after Succelus took the precept of life and was reborn) in order to become the progenitor of the human race. Another immortal creature (Gabella) was created to be the mother of this race, which was to be endowed with equal gifts from all the gods' precepts and was originally intended to be immortal. Gabella refused him, rebelling against the authority of the gods and her husband. She named herself "Gabella the Unbound" (i.e., Os-Gabella), and was hidden by Ceridwen in the Bair of Lacuna. A mortal wife was then made for Nemed. Humanity inherited her physical mortality, but his spiritual immortality. This "divine spark" sets us apart form the animals, and is what allows us the ability to channel the power of the gods themselves.
This has been in the Fall from Heaven History in the Civilopedia since .31 came out.