Transformers: Optimus Prime, Vol. 4

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.