The Heralds were a humanoid race, originally a lesser species native to the planet Iconia. They were genetically-engineered by the Iconians thousands of years ago, to serve them as personal servants and companions. (STO video game: The Iconian War)
Probably the most distinctive feature the Heralds presented, as the Iconians and most species from Dewa III, was the possession of three pairs of eyes, a distinct characteristic from species native to the planet Iconia.
The Heralds were not the Iconians themselves, but both shared a very close common ancestor. The Heralds' similarity to the Iconians can be compared to the apes' resemblance to Humans. Although they were not the powerful energy beings the Iconians were, they also were not simple wild beasts. They were at least as intelligent as any other space faring species. (STO video game: The Iconian War)
After being uplifted by the Iconians to become self aware, the Heralds were developed to have only the male gender. The Iconians' genetic manipulation in the Herald species was very similar to what happened with the Founders and the Jem'Hadar.
Still in utero, the Heralds were modified to fit an specific purpose, and were also imprinted to have devote loyalty to the Iconians. Once born, their incredibly high metabolism allowed them fully mature very quickly and then be extensively trained. (STO mission: "Midnight")
The Heralds were native to Iconia. Thousands of years ago, they were found by the ancient Iconian species, which used them as beasts of burden. When the Iconian civilization reached the age of automation, they used genetic engineering to enhance the Heralds to become intelligent and self aware, becoming personal servants and friends to the Iconian species. A Herald was apparently named after the Iconian they directly served.
After the bombardment of the Iconian homeworld approximately 200,000 years ago, the surviving twelve Iconians adopted a militant and ruthless attitude, and once again genetically modified the Heralds, this time to turn them into soldiers. Although the Heralds were also victims of the bombardment of Iconia, the Iconians had hundreds of centuries to breed them and build their army, so by the time of their return they would have countless fleets to help them retake all that was once theirs.
By the early 25th century, there were probably millions, if not billions of Heralds. At the beginning of the Iconian War, the Heralds were formed the backbone of the Iconian invasion forces and were in command of the Iconian fleets. (STO video game: The Iconian War)
After the death of Colonel Hakeev and his failed attempt to open a gateway to bring the Iconians, an unknown vessel, which was later revealed to be a Herald ship, emerges from an Iconian gateway in the Brea system and captures Empress Sela's flagship, the IRW Leahval. (STO mission: "Cutting the Cord")
A Herald was first seen personally by outsiders when Empress Sela and a group of Alliance officers infiltrated the Andromeda Dyson Sphere shortly before the beginning of the Iconian invasion. As they attempted to escape through a gateway, they are attacked by a Herald Harbinger. (STO mission: "Uneasy Allies")
The Iconian War
At the early days of the Iconian War, the Herald forces raided Starbase 234 and New Romulus. Although Starbase 234 was lost, the Alliance's efforts were capable of handing the Heralds at the planet's surface. Shortly after, Herald forces were responsible for the destruction of the Preserver Archive in Lae'nas III. (STO mission: "Blood of Ancients")
During the Iconians' operations at Calbriden III, the Heralds were personally overseeing the Iconians' plan to create a pocket of subspace to the Beta Quadrant utilizing tetryon particles, which would enable the Solanae to directly aid in the Iconians' efforts. However an Alliance joint mission with taskforce Delta Flight, led by Captain Tom Paris, was able to infiltrate the system and successfully destroy the Iconian station which would be responsible for bringing the Solanae. (STO mission: "Delta Flight")
The seemingly incessant strikes continues when a Herald fleet is sent to attack the Klingon homeworld of Qo'nos. despite many losses, the planet was successfully defended and the Heralds withdrew. (STO mission: "Gateway to Gre'thor; Brotherhood of the Sword")
Heralds tasked to guard an Iconian outpost in the Dinasia system and being personally responsible for the security of one of the Iconians, T'Ket, are unexpectedly assaulted by a group of Alliance officers and House Pegh, the Klingon's secret house led by Emperor Kahless himself. Although the Heralds successfully preserves the outposts' operations, the assault team was able to destroy two Omega particle generators and even injured T'Ket, although Kahless was killed by her. (STO mission: "House Pegh")
Responding to a Vaadwaur distress call, a Herald patrol is engaged and destroyed by the USS Chimera, led by Nog, and other Alliance representatives. Shortly after, the Heralds sent an away team to search for the intruders, however they were able to escape thanks to a Krenim device that allowed them to stay out of normal space-time, functioning similarly to a cloaking device. (STO mission: "Time in a Bottle")
During the Battle of Sol, the Iconians focused all their forces into a full scaled invasion of Earth. Countless Herald ships covered the planet's sky and also the whole system. When the conflict ceased, the Heralds withdrew back to their destroyed homeworld alongside with their Iconian masters to begin the work on rebuilding their civilization. (STO mission: "Midnight")
The Heralds, as well as seen in other races, such as the Dewans, worshiped the Iconians as gods. However the relationship between a Herald and his Iconian master was very close. As their numbers had always been few, the Iconians needed someone to serve as companions and servants. The Heralds performed the routine tasks that kept their society running, and in exchange the Iconians provided them with leadership, technological advancement and protection. Some Iconians considered their Heralds to be their own children, and a Herald would most likely die for their masters if necessary. (STO video game: The Iconian War)