Does The Destiny Cycle feature enemies who become allies against a greater threat?

Yes. The Destiny Cycle begins with Gaelic resistance to Norse occupation and conflict involving Tuatha and Asgardian powers. By Warriors of Destiny, Breanna has forged a blood-bound peace with Odin and bound Loki into that oath. Former enemies can then unite as Norvegrs, Gaels, and Celts turn toward the greater threat posed by Rome.