Used primarily by either Alpha Legion detachments or more commonly amongst Stygian militia. Spawns provided a fast moving shock force throughout the stygian wars. Dark mechanicus biologistic savants under Alpha Legion supervision were able to create spawn more pleasing to the aesthetic values of the Alpha Legion and the Stygian empire. These spawn serpent like in appearance came to be known as stygian spawn. A single stygian spawn often enough spelled death for an imperial guard squad, packs of these creates could even bring down an astartes squad.
I used the angelis model from Hordes as spawn because it fit the serpent image of the alpha legion better, and I liked the model.