The being known only as "Harvester," was one of many serving that role to a race of energy farmers who allegedly existed for "tens of billions of years" (either hyperbole or indicating they originate from a dimension pre-dating Earth's universe, which is estimated to be less than 14 billion years old). According to the Harvester, before Earth condensed out of cosmic ash, the farmers had determined which stars generated the most fertile planets. Almost three billion years ago, the farmers identified Earth, then just a volcanic soup without atmosphere; the oxygen was still trapped in the vast primordial oceans. Via meteors, the aliens rained down parasites that merged with the cells (unicellular life forms) in the oceans, creating something new. Most died, but some spawned generations of host infected with generations of parasites, finally merging into a single creature. Evolution continued, with the parasites eventually becoming mitochondria, the energy factory in all animal cells. Thus the farmers used Earth to develop better mitochondria in increasingly complex forms.
Thirty years ago, the signal was sent to one of the Harvesters that Earth's crop was now ready (marked by the sudden spike in mutant development). The Harvester traveled to Earth, his black transport sphere crashing to earth like meteorite. Landing in Kansas, he was investigated by a curious elderly couple Jeremy and Margo Tenk, and he analyzed their genetic structure, transforming his energy form into physical form suited for Earth, but killing the couple in the process. The Harvester spent the next 30 years casting his net over every mitochondrion on the globe. He made his home in Picayune, Kansas, slowly taking control of the town's residents. Approximately seven years ago, most in Picayune lacked independent will or action, acting out their lives, but doing nothing unsanctioned by the Harvester.
Mutants, however, possessed some inherent resistance, and two years ago, Mike Dorie to escape Picayune, telekinetically carving an extremely immense "Help Us" in the city's croplands. He then fled with his girlfriend Carla Cribb, who remained in blissful ignorance of the problem. Discovering Mike and Carla, the Harvester demonstrated his connection to every living animal (including mankind)'s cells, showing attacks on the Harvester harmed all Earth's creatures. Seeing that he had no hope to stop the Harvester and wishing to spend mankind's remaining time with Carla, Mike returned them to Picayune with the Harvester, who erased Mike's distress message. However, the message had already been seen by alternate Earth-295 ("Age of Apocalypse")'s Nate Grey (aka X-Man), who had made the "mainstream" Earth 616 his home and appointed himself mutant shaman, looking after the world's mutant population. Investigating, Nate confronted Mike (recognizing his energy signature on the message) who reluctantly brought Nate to the Harvester. Recognizing Nate as foreign and unwanted, the Harvester assaulted him, but after a brief struggle, the Harvester revealed how assaults on him would hurt all terrestrial animals. After the Harvester revealed his true nature, origin and intent, he prepared to complete his harvesting, destroying humanity in the process, but Nate - realizing his only option, knowing that his own life was limited and convincing Mike to be his successor as mutant shaman - assaulted the Harvester, converting both into energy, and then merged himself with the Harvester and combined their energy with every living animal on Earth. Both beings ceased to exist as individual entities, but Nate's extradimensional energies tainted the energies of every animal cell on earth, and every cell that would descend from them, rendering the energy unstable to the farmers. Nate was restored to corporeal existence roughly two years later, though his energy taint presumably remained in Earth's animal cells, and the Harvester has not been seen since.
Note: There is no known connection between the energy farmers and the beings known as Sublime and/or That Which Endures, both of which involved early unicellular life on Earth.
