The Fosterlings of the Old Ones are the mutant offspring of matings with human females and Outer Gods or Great Old Ones. Through a special ritual the Outer God or Great Old One sends a dream which reaches into the womb of a pregnant woman, altering the genetic structure of the unborn fetus. Born, the child spends many years as a normal human until one day it transforms into something more closely resembling its alien parent.
After the child begins to resemble its parent, they are either rejected by their human parent, or in most cases being raised further which is due to the fact that the human female is a cultist herself and worships the Great Old One or Outer God that she bore her child with. While they are not as powerful as their parents, they remain to be extremely dangerous nonetheless, so much so that only one could could slaughter a room full of humans.
It is said that the child takes on the personality of their alien parent more so then their human parent. That is due to them being conceived by a being which is beyond understanding, and with an appearance that cannot be defined. A child of an Outer God is actually a very rare occurrence due to the fact that the Outer Gods have no interest whatsoever with the humans, or their worshipers, although Nyarlathotep is an exception.