The Fosterlings of the Old Ones are mutants born to humans whom have mated with a Great Old One or an Outer God. In a special ceremony, the entity projects dreams that reach the pregnant woman's womb altering the unborn fetus DNA. Which lives for years as a normal human, until the day it mutates.
It's transformation happens in one evening, becoming something that more closely resembles its parent deity. Two or more of them may merge into one fosterling. Mutations which are passed down genetically, with each generation becoming more like the alien parent. They are destined to be the chosen servants of the Great Old Ones when they return.