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 deity sends a dream which reaches into the womb of a pregnant woman, altering the genetic structure of the unborn foetus. Once born, the child spends many years as a regular human until one day transforming into something more closely resembling its alien parent. This change occurs in a single evening.

In the RPG...

All the stats given for Fosterlings are fairly generic because their traits will depend on which Old One their parent is. After the transformation, (on average) a Fosterling's STR and CON both gain +80, POW, INT, and DEX gain +25, and SIZ gains +45. They have no armour, but only take minimum damage from weapons.

In a video game...

I think it would really depend on which deity the Fosterling resembled.

In a film...

There is a lot of horror potential from a person changing into a monster overnight. The story that would unfold would depend on whether they knew this change was coming or not.