Protogenos

The Protogenoi were, in Greek mythology, the first beings to exist in the universe and even before the universe was created. A great deal of them weren't really envisioned by the Greeks as having solid forms; instead, they were mainly personifications of their roles in the universe. The first of the Protogenoi was Chaos, who represented the endless void of nothingness that existed before the universe was created. The most well-known of the Protogenoi was Gaea, who represented the earth and her son and husband Ouranos, who represented the sky. The Protogenoi were eventually overcome by the Titans, although since they were personifications of things that could never be destroyed (earth, darkness, etc.), they never really faded. Ouranos was killed by his son, Kronos, and the others, such as Gaea, were merely put to 'sleep' for eons.

The Heroes of Olympus
Gaea, the Protogenos of the Earth, is the main antagonist of The Heroes of Olympus. Much like Kronos in the first series, she and her children, the Giants, are working to destroy the gods; however, they plan on destroying the original Mount Olympus in Greece, since that is where the gods' power is truly held.

Notable Protogenoi

 * Chaos
 * Gaea
 * Ouranos
 * Nyx
 * Tartarus
 * Eros (sometimes)
 * Khronos (not to be mistaken with Kronos/Cronus)