Nephilim

The Nephilim are the offspring of mortals and immortals, they are demigods. However, due to the Nemedians' separation from magic they can not birth Nephilim, such as shown by the Fir Bolg.
They aren't a true race since there aren't many similarities between individuals beyond their common ancestry of having one immortal and one mortal parent. Due to the nature of the Theoi, even siblings can be wildly different in appearance and abilities.

Origin/Ancestry

Immortal + Mortal