The Ring of Winter — James Lowder

As a Harper agent scours the jungle in search of a magical ring, he finds that dinosaurs, a lost civilization, and an evil cult all stand in his way
 
For centuries, adventurers have sought the fabled Ring of Winter, rumored to possess the magical might to make its 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 book in a series of loosely-connected novels about the Harpers.