"Herald of Unicron" originated as a fan term for Transformers who serve, or have been reformatted by, Unicron. It was used in much the same way as shellformer, or as seeker used to be; and like "seeker" has since graduated to (admittedly rare) usage in official sources. Despite its official rarity, fanfictions are rife with it, making it part of the greater fanon. It most likely originates from the term "Herald of Galactus" from various Marvel comics, given the similarities between Unicron and Galactus.
The term's first canonical use was in the Fun Publications fanclub comic where Ramjet refers to himself specifically as the "Herald of Unicron". Later, IDW used the term in Beast Wars II Galvatron's Beast Wars sourcebook biography.
What is a "Herald?"
While many characters are Unicron-related, not all of those characters are generally thought of as "Heralds". While the term originated in fandom, it nevertheless has some boundaries. For example, Galvatron was rebuilt to serve Unicron and lead his "armada" of rebuilt Transformers, which includes Cyclonus, Scourge, and an indeterminate number of Sweeps. All of these characters are sometimes called "Heralds", even though Galvatron wasn't always loyal. On the other hand, Starscream is not considered a "Herald", due to his complete lack of loyalty. Starscream never intended to serve Unicron and abandoned the Planet Eater after getting his reward.
Another, more appropriate example would be Sideways and the Fallen. Both are almost universally considered Heralds of Unicron among fans for the same reason that the Silver Surfer was considered a Herald of Galactus; their appearance actually signified the appearance of the Chaos Bringer, as they were trying to bring him back into existence. Sideways wanted to lead Unicron to the Transformers and destroy Cybertron, while the Fallen was attempting to free the Planet Eater from his extradimensional prison. Hook, Line, and Sinker served a similar purpose, as their arrival at a particular world meant the Chaos Bringer would soon arrive to devour everything.
The insignia of the Beast Wars Neo faction, the Blendtrons, has been associated with the "Heralds". Initially a fan convention based on the Blendtron origins and the general look of the symbol, the glyph has since entered official usage; the Ark Compendium used it to represent Unicron (though not Galvatron, Cyclonus or Scourge), and the mark adorned the Kiss Players Sparkbots after their allegiance to Unicron is revealed. Some custom sticker suppliers have developed faction sigils and rub symbols with the Blendtron logo as a "Herald of Unicron" faction symbol for characters who serve Unicron.
Alternatively, the Mini-Con logo has been used in-show (particularly in Energon) as Unicron's insignia. This is complemented by the M shaped frame around his fame/helmet, which is nearly identical. This connection was meant to hint at a connection between the Mini-Cons and Unicron before their origin was revealed (like, two episodes later), and was further built on in Energon during his one-and-only transformation sequence.
Known "Heralds of Unicron"
Following is a short list of characters from various continuities that served Unicron, whether by free choice or by having their will stripped off of them like a banana peel, and may thus be considered "Heralds":
- The Blendtrons
- Nemesis Prime
- Alpha Quintesson
- The Terrorcons
- Dead End
- Aqua Raider Mini-Con Team