Heralds resemble bipedal toads with 6 eyes and two mouths.
Little is known about them, but they were apparently a very old race. Long ago, they attempted to resist being destroyed by the Eternal Ones, but were defeated by the previous servants of the Eternal Ones. Luckily for them, they were given the choice to become the new Heralds of the Eternal Ones, much like other races before them. They aren't really called Heralds, its a name given to them by their masters the Eternal Ones, because they herald their arrival. In fact, there have been several Heralds before them. Besides heralding the arrival of the Eternal Ones, they forced other races to comply with their master demands, or the Heralds attacked them. They did not do so because they wanted to, only because it spared their race from destruction.
When the Eternal Ones begin to arrive in the Kessari Quadrant to feast on the local sentients, the Heralds were sent to keep order as their masters feasted. The Human Captain with his Precursor Ship and a fleet of League ships managed to defeat the Heralds, despite the strengths of their ships. The Heralds were impressed, they had never been defeated before. However they knew this alone wouldn't slow down their masters. However the Human Captain convinced the Herald Captain to join forces with the League to crush the Eternal Ones. In the nick of time, the Humans learned the Eternal Ones could be fed without the destruction of all sentient life, sparing the univers. It's assumed the Heralds joined the League and rebuilt their empire.