Gateways, The Evertrw Adventures Book 3:

Thirteen gates. One unicorn horn. And a final choice that will change everything.In Gateways, the conclusion to The Evertree Adventures, Robert and his friends face their greatest challenge yet: opening the Thirteen Gates, the only way to end Sunnhet's dark power and free the unicorn horn once and for all. But Sunnhet has raised an army to stop them, and Time is not on their side. In fact, Time is trying to stop them, too.Their journey carries them across continents and through ages long past. Along the way, they gain help from a gargoyle who's pretty much afraid of everything, a group of Viking warriors who enjoy a good quest, and a mob of Samurai ghosts that will go to any length to stop Sunnhet. But as the final gate nears, the cost of victory becomes clear. The world can be saved, the horn can be freed, and the gates can be opened-but only if Robert gives up the one thing he treasures most: his friendship with Joe Evertrw.A sweeping adventure filled with courage, heartbreak, and the weight of impossible choices, Gateways delivers a powerful and emotional finale to a series where hope, loyalty, and belief shape the fate of worlds.