Reason: http://en.wikipedia.org/wiki/Elements_of_the_Cthulhu_Mythos
"Fosterlings of the Old Ones: 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."
Updated by FoxFourOhFour