In the fourth live-action Transformers film, Transformers: Age of Extinction, Prime departed Earth to seek out the Transformers’ creators – but based on The Last Knight trailer, he appears to have suffered a fatal setback. It’s possible that he could have been transformed into a zombie; in fact, that same thing happened in the animated TV series after Prime’s famous death in the animated Transformers: The Movie.
It’s also possible that Prime could be operating under the influence of Unicron. A planet-sized Transformer who devours other world’s for fuel, Unicron figures prominently in several different continuities of the Transformers mythos (sometimes as the opposite number of Autobot “god” Primus) and is often depicted as rebuilding dead or injured Transformers to become his servants. That kind of behavior is also associated with The Quintessons, who in some tellings are the very “Creators” Prime had gone to investigate at the end of Age of Extinction.