Festerlings are humans, or the descendants of humans, who have been genetically tainted by DNA analogs of Suthidruu genetic material. This means that they do NOT have mixed human/Suthidruu DNA (that would be silly, and biologically unfeasible), but those genetic sequences found in Terran DNA that result in some of the same characteristics as their creators and masters. Suthidruu genetic material uses a different molecule than Terran life, and so is incompatible.
The one set of traits all festerlings have is the inborn Suthidruu tendency for fanaticism, all sharing the same devotion to the Suthidruu theology, and to the Suthidruu themselves. This means that all festerlings work to achieve the goal of the total extinction of all life in the salvation of its souls for paradise. The same factor results in their reluctance to simply send themselves to paradise and be done with it: They are not worthy, until their masters tell them otherwise. Festerlings never commit suicide except to avoid capture, and only upon command from a Suthidruu. Murder, though is a different story, as like their masters, their killing is motivated by love and compassion, not hatred or anger.
This is like some instances during the various Inquisitions of the Medieval church on Terra, when victims were tortured and executed, usually by burning at the stake, out of love and concern for the well-being of their souls. In other instances, accused heretics were simply tortured and eliminated as spiritual damage-control, to serve as an example for those contemplating heresy.
Festerlings also have more physical traits, such as tentacular limbs and appendages like those of their masters, but usually concealable in the case of those serving as spies among humans. Usually, though, spies are chosen from those who are completely human-looking, and those with malformations serve as menial labor and shock-troops in assaults on human colony worlds. As long as the human-looking festerlings avoid a medical examination or DNA scan, they are difficult to detect.
Festerlings have already infiltrated human worlds, and lead cults dedicated to the Suthidruu, who serve as proxies for their Gods, and all working for the total universal genocide they must carry out to bring on the Eschaton, and escape from this universe of pain and sorrow. Needless to say, they are a threat, and often a subtle one, so look for flash fiction featuring them on this blog and elsewhere.