I read a theory years ago about Craster being descended from the WWs. Or the Night's King, perhaps. Mostly tied to his sacrificing his kids to the Others. Why would they come all the way to Craster's place to collect
his kids when they seemingly could have just plucked them from more Northern wildlings? What was so special about his kids? And might it relate to his incestuous relationships, similar to the Valyrians inbreeding to maintain pure blood?
Also interesting the descriptions of him from Ygritte. Also of note that Dywen said Craster had a cold smell, which was consistent with how he described the smell prior to the Wight attack at the Fist of First Men.
"Craster's more your kind than ours. His father was a crow who stole a woman out of Whitetree village, but after he had her he flew back t' his Wall. She went t' Castle Black once t' show the crow his son, but the brothers blew their horns and run her off. Craster's blood is black, and he bears a heavy curse."
Keeping in mind that the Night's King as written isn't exactly what we're seeing on the TV...
Night's King lived during the Age of Heroes, not long after the Wall was complete. He was a fearless warrior named the thirteenth Lord Commander of the Night's Watch. Later, he fell in love with a woman "with skin as white as the moon and eyes like blue stars". He chased her and loved her though "her skin was cold as ice", and when he gave his seed to her he gave his soul as well.
During the dark years of his reign, horrific atrocities were committed, of which tales are still told in the north. It was not until Brandon the Breaker, the King of Winter, and Joramun, the King-Beyond-the-Wall, joined forces that Night's King was brought down and the Night's Watch freed. After his fall, when it was discovered that Night's King had been making sacrifices to the Others, all records of him were destroyed and his very name was forbidden and forgotten.
Could be that Gilly's kid has some role to play when his cousins come calling?