Darkness falls over Cybertron as the legendary Onyx Prime returns, throwing the planet into chaos! What does this mean for Optimus Prime--and Earth? As Onyx and his comrades sow panic and dissension on Cybertron, Windblade defends an already tense planet against a secret that could shatter it forever, while Soundwave questions a fragile alliance, and Optimus confronts a mystery that goes back millions of years. Collects Transformers: Optimus Prime issues #15-21.