War of the Griddons. Ambassadors of Orealis, #3

The final book of the Ambassadors of Orealis trilogyTwo worlds saved. One final battle. The Ambassadors of Orealis aren't done yet. While visiting a friend at the hospital, Kalli Kristjansson overhears a name that stops him cold: Gunnar Ebbason, his mysteriously long-absent grandfather. Compelled to visit the man he's never met, Kalli finds Gunnar ranting about creatures called Griddons and the Queen of the Hidden People, someone straight out of fairytales.
When Gunnar suddenly collapses, Kalli finds himself pulled into a world he never knew existed. The Hidden People of Álfaborg call on the Ambassadors of Orealis as hundreds of crystalline eggs, half-buried on a black-sand beach, are not only stirring, but the first hatchlings are already feeding and growing fast. Worse, they're calling to an ancient queen through a weakening portal. Untrained and shocked by his own ancestral secret, Kalli finds himself linked to these creatures' minds and must learn to control the terrifying connection.
Along with his friends, Finna and Erik, they rush to stop these dragon-like creatures before the whole world becomes food for an invading army. The clock tightens as the largest Griddon nears the old portal site. One more call could bring an entire army over, ready to feast on Earth's inhabitants. A fast-paced fantasy adventure about secret bloodlines, impossible courage, and one teenage boy holding together the fragile seal between worlds-until he might have to pay the ultimate price.