Before the idea of heralds, Galactus created a self-replicating program that sought out planets for him. The Cosmos Automation Program would possess someone of a planet to create a machine that would store cosmic energies and vanquish any threats to his consumption of the planet. After several planets learned how to turn their creation against Galactus himself, he searched for new means to help him sustain his hunger.
Heralds of Galactus
In order to efficiently locate worlds with which to feed, Galactus will employ beings known as his heralds whose job it is to seek out worlds that will sustain him and satisfy his insatiable hunger. Heralds are imbued with portion of Galactus' power, an energy known as the Power Cosmic.
The Power Cosmic gives his heralds special abilities often including the ability to survive in the vacuum of space, limited invulnerability, faster than light travel, and the ability to generate massive concussive blasts. Not all heralds are created equal though, and vary in power.
Many of Galactus' heralds wield weapons that can channel their cosmic energies. These items include the Silver Surfer's board, Firelord's staff, Stardust's staff, Red Shifts swords, and Terrax's axe.
Generally, Galactus has only one herald at a time.
List of Heralds in Earth 616
During the events of the Annihilation, Galactus accepted the Silver Surfer as his herald once again. After the Annihilation, Stardust was proven worthy again by feeding the last of his race to Galactus. Because of his capture during the Annihilation, Galactus' hunger rages more than ever. Also because of Stardust's unparalleled display of allegiance and Galactus' knowledge that eventually the Silver Surfer will betray him again, Galactus has allowed Stardust to carry on as a second herald with the Silver Surfer.
While Stardust has been inexplicably absent from any recent events, the Silver Surfer was recently relieved of his duties and replaced with the Earth man who would come to be known as the Praeter.
It is revealed to Superman that his home world, Krypton, was destroyed by Galactus, a planet-consuming entity that has existed since the dawn of time. Upon learning about the true fate of his own world, Superman travels to the Marvel Universe to seek the help of the Fantastic Four in putting an end to Galactus' universal threat. Before the team has a chance to fully brief Superman about Galactus , Galactus tracks down Superman and turns him into his new golden herald. In the end, it isn't until a deal is cut with Cyborg Superman that Superman is finally saved from Galactus' control.
In the Heroes Reborn world, Galactus has five heralds. They are the Silver Surfer, Firelord, Terrax, Air-Walker and Plasma. They were all used to try to consume the Earth, but were eventually unsuccessful when the Silver Surfer turned on his master.
In the Earth-691 reality, Nova is herald until the mid 30th century until she is killed sacrificing herself to save Galactus after he once again came to Earth. Two of those involved in her death were former heralds Silver Surfer and Firelord. After this, Galactus vowed to never have another herald.
Galactus had an Alien Silver Surfer, although it was indicated that Norrin Radd was also the Silver Surfer at some point in this universe. After the death of the alien, Hercules' granddaughter Juno became herald, also to be made in the appearance of the Silver Surfer.
In the Earth-M reality, the Silver Surfer rebels against Galactus on his own when he comes to Earth. Galactus creates a new herald Ardina but she is later undone by her creator. At some point, Galactus choses another herald known as Dark Angel.
Earth-20051 (Marvel Adventures)
Primordius, a previous Herald fired by Galactus for being insufficient, attacks the Silver Surfer out of jealousy. Primordius served "eons ago," so presumably he came after the Fallen One (if the Fallen One exists in the 20051 universe) and before the Surfer.
Marvel vs Capcom 3
In Marvel vs. Capcom 3: Fate of Two Worlds, Galactus appears as the final boss. However, before Galactus can be fought, the player must first defeat two of his heralds. Both heralds appear at once, but they each share a single life bar. There are four possible characters that can appear as heralds: