The White Thread: The Gateway Chronicles 3

Darcy Pennington spent her freshman year agonizing over the sacrifice of her dear friend Yahto Veli, the nark who gave himself to the Oracle to set her free. But when she returns to Alitheia determined to rescue him, she soon comes to realize there's much more at stake than just the fate of her friend. Alitheia is in turmoil, and a new riddle from the Oracle hangs over their heads. The Six, the alchemist Rubidius, and Prince Tellius must set off on a sea voyage into the realm of a legendary...