As a force of nature, Galactus must consume planets for the energy and sustenance he needs to carry on his tragic existence. Over the eons, Galactus has taken on Heralds to aid him in the finding of those worlds. The Devourer grants each Herald a portion of the Power Cosmic to aid them in their efforts.
According to Thanos' research, the Power Cosmic creates a different kind of being every time because the form it takes follows the function of the individual. The Devourer can, and has, guide the results, though. The energy imbued by Galactus fuels the Herald's life force, which allows for them to survive unaided in space, not needing to consume anything themselves or rest unless incredibly depleted of energy.
These beings are often met with hostility, so their powers also have defensive and offensive capabilities. Many also use a weapon to channel their cosmic energy. Over the eons, Galactus has tried a number of different Heralds, all with different personalities who have served him to varying degrees of success. He often plays with their minds and memories to keep them docile for as long as possible. In most cases, even the Heralds that Galactus severs ties with are allowed to keep their powers and live. It is also nearly impossible to permanently kill a Herald.
The Failed Prototype
The first known Herald of Galactus is called Fallen One. Instead of being granted Power Cosmic, he was fueled by Dark Energy. Very little is known about how he and Galactus worked together, though Fallen One lost his position when he reveled in the destruction too much. Galactus jailed his servant, but Fallen One broke out and attempted to kill his former master.
This cycle of imprisonment and attempted vengeance has gone on for thousands of years until recently. The Devourer was so distraught by this failure that he not only buried most evidence of Fallen One's very existence, but also held off on bringing in another Herald. Before the Annihilation Wave hit, Thanos took control of Fallen One and made him his own Herald. However, the first Herald was taken out by Galactus' ancient enemies Aegis and Tenebrous.
The Silver Sentinel
Eventually, Galactus made his way to Zenn-La, a planet that he intended to absorb. However, one of the denizens, Norrin Radd, offered himself up to Galactus as a servant if the Devourer would spare his world. Radd was transformed into a being now known as the Silver Surfer and served his master faithfully.
However, everything changed for Norrin when he guided his master to the planet Earth. Upon meeting the Fantastic Four and Alicia Masters, the Surfer decided to betray his master and side with the humans! When the Human Torch managed to get his hands on the Ultimate Nullifier, the Earthlings brokered a deal that would keep Galactus away from the planet going forward. The Devourer agreed, but banished the Silver Surfer to live on Earth instead of surfing the spaceways, though that would prove far from their last meeting.
The Daring Xandarian Duo
Of Armor And Axe!
While serving Galactus, the enchanted Asgardian armor managed to find Counter-Earth. That world's creator, the High Evolutionary, attempted to hold off the Herald alone, but was soon assisted by the Fantastic Four. Galactus arrived and sent the Destroyer back into his ship, facing these powerful beings himself. At that point, Loki used his powers as the new king of Asgard to steal The Destroyer away from the Devourer!
Unlike the other Heralds, Tyros was not asked if he wanted the job and was transformed into Terrax the Tamer against his will! Galactus explained that he gave Tyros the power over earth and rock to carry on a theme of Heralds with elemental abilities noting that the Surfer represented the seas, Firelord fire and Air-Walker air.
With Terrax, Galactus also went a different route by intentionally going after a cutthroat being who reveled in conquest. That resulted in an attempted rebellion right after his rebirth. Though Terrax learned the full extent of his master's power at that moment, he never stopped looking for ways to unburden himself of his new role. Eventually, Terrax realized that he could starve Galactus before getting him near Earth where he would force the Fantastic Four to kill his master or watch Manhattan be destroyed. Though Galactus stripped Terrax of his power and sent him falling towards Earth, the Tamer has since regained his powers with some help from Doctor Doom.
Forged In Fire
At the time of Terrax's gambit against Galactus, Earthling Frankie Raye had been hanging around with the Fantastic Four and getting a handle on her fire-based powers. She witnessed the grand power of The Devourer firsthand and offered herself up as his new servant in an effort to save the Earth much to the dismay of her boyfriend Johnny Storm.
Nova understood her master's needs and had no compunctions about guiding him towards inhabited planets. While serving as Herald, she felt something like love towards Galactus, but also began a relationship with the Silver Surfer who kept trying to push her towards offering Galactus planets without life. Nova notably lead Galactus to consume the Skrull homeworld Tarnax IV, an action with repercussions still echoing through space to this day.
After a trip through time, Nova finally began to understand the Surfer's pleas for compassion when selecting planets. However, when she attempted to deceive her master about suitable celestial orbs, he punished her by making her watch as he approached a planet with living beings and also chose her successor: Morg the Executioner.
Tired of his Heralds having moral compasses, Galactus pivoted once again in choosing Morg. Completely bloodthirsty and loyal only to power, Morg proved a devoted Herald, though he still worked to amass more personal power for himself. Upon meeting Galactus' new hire and fighting him, the Silver Surfer realized that Morg could lead to the deaths of millions and intended to stop him with the combined might of fellow Heralds Firelord, Nova, a rebooted Air-Walker and Terrax.
While the others battled fiercely against Morg, Silver Surfer flew off to try and reason with Galactus. During the fight, the current Herald killed Nova and dealt Air-Walker catastrophic mechanical injuries. After talking with Norrin, Galactus conceded and removed Morg's Power Cosmic, which lead to his death at the hands of Terrax. With Galactus Heraldless, Firelord and Air-Walker agreed to take up the position together, doing their best to steer the Devourer to uninhabited planets. Though the world-eater agreed, he also secretly resurrected Morg who went after Terrax to get his Cosmic Axe back. This move lead to his capture by Galactus' ancient enemy Tyrant.
At some later point, Galactus became addicted to consuming the life force of the beings living on the planets, which was not sustaining him, but instead poisoning him. By then he brought on a new Herald, Red Shift, who brought him to Earth once more. However, in battle with Silver Surfer and Alicia Masters, the new Herald was left behind in a black hole. After Earth's heroes attempted to fend off their attacker, the Surfer agreed to be Galactus' Herald once more.
Leading Galactus to the Shi'Ar homeworld, the Surfer managed to turn Galactus' own machines against him. Given his already weakened state, the Devourer fell and was converted into celestial energy, effectively ending his life. Before dying, though, he told Norrin Radd that he chose him all those years ago to facilitate this very moment.
The Human Element
Franklin and Valeria Richards would eventually bring Galactus back to stop the reality-threatening Abraxas. The rejuvenated Devourer managed to fend for himself for a time, but returned to Earth once more when a being called Zius intended to kill the Invisible Woman because she alone could work around the machines he had built that would cloak planets from the Devourer. Zius intended to kill Sue, so Reed Richards switched her powers' for her brother's. Galactus appeared on Earth killed Zius and instantly transformed Johnny into his new Herald!
Not wanting to be turned into a murderer, Johnny struggled to find uninhabited worlds, but kept finding various lifeforms of all shapes and sizes. Ultimately, the Fantastic Four and Quasar showed up and helped Storm figure out a way to turn Galactus human for a brief time, though the human lost his Power Cosmic in the process. A trip to Earth gave Galactus a newfound respect for Earth and life before he returned to his great cosmic mission.
Riding The Annihilation Wave
Galactus managed to find a new Herald called Stardust who considered those about to be devoured by him blessed. She lead him to New Korbin, the recently established home of Beta Ray Bill's people. The ensuing battle between the new Herald and the Korbinite warrior briefly unleashed a space demon, but the pair went their separate ways with Stardust continuing to serve her master.
Shortly after, Annihilus decided to leave the Negative Zone and declare war on the Positive Matter Universe. He sought to collect as much energy as possible and took a specific interest in the Power Cosmic, even going so far as to send his Seekers out to attack and capture Galactus' former Heralds. One such attack lead to the destruction of Air-Walker. Another brought Firelord and Red Shift together. The Silver Surfer appeared too late to save his android brother-in-arms, but became aware of the true threat.
Hanging Ten Again
The Surfer alone attempted to feed his master Sakaar in hopes that the Old Power would sate him for a hundred thousand years. His plan to evacuate all living beings from the planet first was foiled when he was beaten by Skaar and fitted with a power dampener. Eventually Galactus did come to Sakaar, though, and consumed the planet. Before being transported away, though, Skaar managed to infect the Devourer with an intense craving for more Old Power. Knowing what he wanted, Galactus briefly severed ties with the Surfer during an addiction-fueled rage.
Eventually free of his craving, Galactus reunited with the Silver Surfer to fight against the Cancerverse's invasion as well as the threat posed by the Chaos King. The pair also became aware of a Galactus corpse buried on Earth. Reed Richards explained that a group of people on a dying future-Earth used the Devourer's power to travel back in time to survive. It worked, but future-Galactus died and they wound up on a planet called Nu-World. The Devourer sought revenge against his future murderers, but also wanted to find out what could kill him. Reed figured out how to save the people and get everyone back home.
The Silver Surfer found himself arriving on Earth once again in advance of his master when the World Heart--otherwise known as a Galactus Seed -- had been located in Asgardia, which then floated near Broxton, Oklahoma. The Herald tried asking for the seed, but Odin and Thor denied him vehemently. When war with the Asgardians proved potentially deadly and Loki managed to hide the seed, Odin and Galactus agreed to a truce that involved the Surfer sticking around to keep it safe. In his place, Mike of Broxton took his place as Herald and became The Praeter. Praeter was never seen again after that initial meeting.
In a bit of role-reversal, a Franklin Richards from the future used his younger self's incredible power to bring Galactus back from the dead, making the Devourer his own Herald! Galactus assisted in removing the threat of Celestials before returning to normal.
A New Kind Of Herald
Galactus did not have a Herald when he made his journey into the Ultimate Universe, but eventually made his way back thanks to some assistance from The Eternals. He received a very different kind of assistance from a new group called The Ultimates -- Blue Marvel, Captain Marvel, Black Panther, Ms. America and Spectrum -- figured out how to finally rid Galactus of his hunger changing him into Galactus, the Lifebringer!
His new mission then involved rejuvenating planets, many of which he had consumed previously. After Conner Sims, also known as Anti-Man, was tricked into breaking Thanos out of the Triskellion, he took off and wound up with Galactus who turned him into the First Herald of Life! For his task, Anti-Man was sent by his new master to Earth to recruit the Ultimates to find a way to unshackle Eternity!
Anti-Man succeeded in his task and the Ultimates agreed to also become Heralds! They all worked together to fight The Eternity War against The First Firmament, the original incarnation of reality! One of the earlier conflicts included saving Galactus who had been made to hunger once more by a new cosmic force called Logos. Anti-Man sacrificed himself to return Galactus back to the Lifebringer. From there, the Ultimates handled threats on one level while Galactus worked with the Eternity Watch -- Ego-Prime, Psi-Hawk, the ghost of the Shaper of Worlds and The Infinaut -- to release Eternity and save reality! Galactus and the Ultimates left on friendly terms.
The Hunger Returns
Galactus' time as the Lifebringer came to an end because of a request made to him by his former Herald the Silver Surfer. When the Infinity Stones were reborn, all manner of beings sought them out. On Saiph, Ultron acquired the Soul Stone and planned on using it to send drones of himself throughout the cosmos which would further his plans of destroying all organic life. To stop the destruction of countless worlds, Galactus agreed to consume Saiph which transformed him back into The Devourer. Though this meal provided plenty of energy, Silver Surfer offered his services once more as Herald.
After being briefly overpowered by Doctor Doom on Earth, Galactus was shunted into the Mystic Realm. Concerned about what this would mean for existence, Doctor Strange followed him and prevented the Devourer from consuming the wrong magical energies, Strange offered himself as Herald. Strange inadvertently lead Galactus down a path that would briefly unite him with Dormammu! Eventually, the Master of the Mystic Arts received assistance from a group of heroes including several surviving Heralds like Firelord, Air-Walker, Nova, Silver Surfer and Terrax who, along with Strange, managed to get Galactus back in his proper realm.
Herald Of The Storm
When Thor became All-Father after the War of the Realms, Galactus came crashing into Asgard. Nearly dead and mumbling about the Black Winter, Thor called those associated with the Devourer for information including Silver Surfer, Nova, Stardust, Alpha Ray (who was changed by Galactus during Stardust's first encounter with Beta Ray Bill) and Cosmic Ghost Rider (a Herald from a possible future). The Surfer explained that the Black Winter destroyed Galactus' original reality, but that he had found five planets that would offer his former master the power he would need to survive meeting it once again. They all agreed to help, but Galactus instead chose Thor as his new Herald!
Before the world-eater could consume the first of these power planets, Thor called for a Bifrost Bridge to carry the main inhabitants away before Galactus devoured. After a brief conflict with Beta Ray Bill, Thor and Galactus continued their rocky partnership harvesting the energy of three more worlds without killing anyone. However, the Black Winter caught up to them on the fifth, which Galactus quickly consumed in preparation for battle.
The Winter, taking the form of a dark Thor, revealed that he had not come to destroy reality, but instead to kill his own Herald, Galactus! Going further, Winter said that he had spared Galan of Taa so that he would absorb energy that Black Winter would eventually come to collect. Furious, the real Thor absorbed all of the power Galactus had amassed from the special planets and then executed the Devourer with mighty Mjolnir. Thor then used all of Galactus' stored energy in an attempt to destroy Black Winter. Though the mighty devourer has fallen and many planets can breath a sigh of relief, a primal force such as him will likely not last for long.