La ruta de las abejas / Jorge Galán.

Enhanced descriptions from Syndetics:

Lobías Rumin lives in Eldin Menor, and unlike the rest of its inhabitants, he is a strange and lonely boy. But Lobías' life takes an unexpected turn on the eve of the spring equinox, when he meets Nu and Lóriga, two travelers with the intention of carrying out the most insane undertaking possible: crossing the Valley of the Mists. The mist there never dissipates and for centuries no one has dared to cross it. Travelers say they follow the instructions in an old book to find a path that accompanies the route of the Morneas bees. Thus, reach the Homa Tree, the first of the world's trees. Without stopping to think twice, Lobías accepts his invitation, but what the expedition finds on the other side of the mists is as strange as it is terrible: a world at war where entire peoples flee from an army that arrives from distant lands and advances, devastating everything in it's path. The route of the bees tells the story of a journey, but also that of a war against sinister forces. For Lobías Rumin it will mean meeting a destination that he would never have imagined, an adventure that will change his life forever.