It's hard to believe this is the third full week of the A-Z Blogging Challenge. To celebrate, Chaos Season is on sale this week for $0.99.
"Nils" is an insult Salth and her son Sal-thaath call people who can't use magic. Although ordinary people can't use magic, they do have some magic associated with their souls. (This is what allows Gwen to see people's auras and tell what their birth seasons are.) Salth realizes that since there are so many more Nils than magic-users, she could gain a fair amount of magical power if she could harvest the small amount of magic each of them has.
This term only crops up in Seasons' Beginnings.