What I gathered from the GB quick look:
Sonic and friends (MC and Cortana) are chased into a tomb (Requiem) by their long-time foes, Eggman and Metal Sonic (The Covenant), and think that turning on some beams (turning off radio interference beams) in this tomb they're now trapped in will help them escape. Instead, Sonic's activation of the beams awakens this "Ancient" (Forerunner) who has been imprisoned there for 1000 (100,000) years. The Ancient, named Lyric (Didact), blames Sonic, or a look-alike, ancient ancestor of Sonic's species, as wall carvings would indicate (humans), for his imprisonment. Sonic tries to stop him, but Lyric flies away. A bit later, Sonic finds out that the Ancients, whose goal was to live in harmony with nature, collectively decided to imprison Lyric after discovering his plan to use magical crystals (Composer) to power an army of machines (Prometheans). They believe Lyric intends to reboot his army and take revenge on those responsible for his punishment.