Sword in the Stars by Amy Rose Capetta

In this epic sequel to Once & Future, to save the future, Ari and her Rainbow knights pull off a heist… thousands of years in the past.

Ari Helix may have won her battle against the tyrannical Mercer corporation, but the larger war has just begun. Ari and her cursed wizard Merlin must travel back in time to the unenlightened Middle Ages and steal the King Arthur’s Grail—the very definition of impossible.

It’s imperative that the time travelers not skew the timeline and alter the course of history. Coming face-to-face with the original Arthurian legend could produce a ripple effect that changes everything. Somehow Merlin forgot that the past can be even more dangerous than the future…