Until You Return - Chapter 1 - needs_sleep_and_food - Transformers - All Media Types, Transformers (Bay Movies) [Archive of Our Own]
Summary:
A new era has begun. And the age of the Transformers...is over.
One false distress signal.
One ruthless bounty hunter.
That's all it takes.
When a false distress signal lures Optimus Prime into Lockdown's trap, he and Megatron are torn apart, each later believing the other was killed. While Megatron wages a war between grief, loss, and the ever persistent Lockdown and army of humans out for him, Optimus is rescued by Cade and his family that get swept up in the run from Lockdown and humans.
Admist the gunshots, screams, explosions, and fear, Optimus and Megatron never stop fighting until they find one another once more.
(Prologue can be found here)