The Harbingers are Crogenitors that, like their Destructor Counterparts, are transfected with E-DNA. However, unlike Destructors, Harbingers are transfected with the Stabilized Variant used in Genetic Heroes.
Also unlike Destructors, Harbingers are still considered 'true' Crogenitors, as they have not betrayed the Crogenitor Insurgency to the Darkspore, like the Destructors did.
One known sign that a Crogenitor is a Harbinger is that they appear to be made up of a combination of Genesis Types, rather than a single one. They tend to be Hybrid Genesis Types, therefore, although combos of 3 or more have been observed.
Crogenitor Xylan, aka the Corrupter, was the first Crogenitor to use E-DNA on other Crogenitors. However, he wouldn't be the last to experiment on Crogenitors using E-DNA. A thousand years later, the Crogenitor Zodark, the Precept Keeper, realized the need for the Crogenitors to join the fight. So, he transfected himself with the E-DNA meant for his genetic heroes. However, like the Genetic Heroes, his power was merely boosted, and, since the E-DNA was stabilized, he wasn't mutated like the Destructors were.
With this revelation, he sent his findings to the surviving Crogenitors: they may have to end up transfecting themselves if they are to survive. Each Crogenitor took up a new moniker to match their new powers: Zodark, the Precept Keeper, for example, became known as Zodark, the Cybernetic Revenant.