Following the recreation of Cybertron as a technorganic world, Unicron began gathering Transformers from across the multiverse, forcing them to fight and harvesting the Sparks of the losers to restore his power. With each spark devoured, Primus was weakened, and so, communing with Autobot elder Alpha Trion, called back from the Allspark the now-legendary Maximal commander and Cybertron's savior, Optimus Primal, to rescue the possessed, battling captives from the Pit of Unicron -- and to lead a team of multiverse warriors to put a stop to his soul-stealing scheme before it could cause the Allspark to collapse.
Primus allowed Optimus Primal to choose any Transformer who ever existed -- living or dead -- for his army. This required considerable amounts of Primus' power, so Primal was told to choose sparingly. Primal chose his departed former troops Rhinox and Depth Charge. Ultimately, however, Unicron's defeat came not as a result of their efforts, but as a consequence of another battle, happening in another corner of the Transformers multiverse.