The Heralds were originally a lesser humanoid species native to the planet Iconia. The Heralds were "enhanced" by the Iconians after the fall of the first Iconian Empire 200,000 years ago to serve the Iconians as personal stewards and guards.
The Heralds are not the Iconians themselves, but they probably share a common ancestor with the Iconians. The Heralds similarity to the Iconians can be compared to the apes' resemblance to Humans. Like the Iconians and the natives of Tau Dewa, they too have 6 eyes. The Heralds' relationship with the Iconians runs deep. It’s more symbiotic than master/slave. Although they are not the powerfull beings the Iconians are, they are not wild beasts. They are at least as intelligent as any other space faring species. (STO video game: The Iconian War)
The Heralds are native to the planet Iconia. Before uplifted by the Iconians, the Heralds were a lesser species of Iconia - probably one who fell behind in technology, or perhaps they were pets at one time.
By the early 25th century, there were probably billions of Heralds. Although they were too victims of the bombardment of Iconia, the Iconians have had hundreds of centuries to breed them and build their army. By the beggining of the Iconian War, The Heralds 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 raid 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 missions: 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)
The Heralds, as well as seen in other races, (the Dewans, for example) believed the Iconians to be their gods. The relationship between a Herald and his Iconian master was very close, as they would likely even die for their masters if necessary. (STO video game: The Iconian War)