From Sonic Retro
The Precursors are an extinct race of beings mentioned in Sonic Chronicles: The Dark Brotherhood. According to Nestor the Wise, the Precursors lived in the Twilight Cage long before the Kron, Voxai, Zoah, N'rrgal, or Nocturnus arrived.
Sonic's party can uncover ancient Precursor tablets during their explorations of the Twilight Cage, often as the reward for completing a side-quest like exposing Haniman's Night Stalker fraud or solving Croesus's psychic isolation. With Nestor's help translating the ancient script, the team learns about the Precursor's theories on the nature of Argus and the Twilight Cage as a whole.