The Ring of Winter (Forgotten Realms: The Harpers, #5)

From the cover:
For centuries, adventurers have sought the fabled Ring of Winter, rumored to possess the magical might to make the wearer immortal and bring a second Ice Age down upon the Realms. Artus Cimber knows where it is.

After discovering the ring is hidden in the jungles of Chult, he sets off to fulfill the quest that has devoured a decade of his life. Knowing that the artifact is hidden somewhere in the danger-filled jungles and recovering it are two entirely different matters, however--especially when a lost city, rampaging dinosaurs, and the villainous Cult of Frost all stand between Artus and his goal.

The Ring of Winter is the fifth in an open-ended series of novels focusing on the Harpers, the secret organization for Good in the Forgotten Realms® fantasy world