Dead Energon is Energon that has gone stale from lack of use...also known disdainfully as Corpse Energon as to how it is generally retrieved.
Reports are claiming that the inhabitants of Silent Grill have harvested this from the fallen of this brutal six-eon war.
Dead Energon, or stale energon if you prefer its proper name, is a remnant of its former self. Just enough power to force the pumps and valves of Cybertron to function, and not enough to make it sustainable. In short, it'd force Cybertron to burn itself out. The sea has been alluded to have been collected from all of the dead that had fallen in this six eon war...and the dead of Cybertron are claiming their toll upon the living.