Darkness falls over Cybertron as the legendary Onyx Prime returns, throwing the planet into chaos! What does this mean for Optimus Prime—and the 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, Soundwave questions a fragile alliance, and Optimus confronts a mystery that goes back millions of years. Collects Transformers: Optimus Prime issues #15-21.