Drowned Kingdom, The (Book 4) - Impossible Quest Series

Four unlikely heroes are forced on a perilous quest to save their people. But first, Tom, Elanor, Sebastian and Quinn must escape the overrun castle. Only they can find the four impossible elements needed to wake the sleeping warriors of the legend. Only they can drive back the darkness that reigns over the land.

I’ve heard of a kingdom that was flooded and drowned...
And that is where the sea serpents live...

Quinn, Sebastian, Elanor and Tom have found a dragon and conquered the dreaded Beast of Blackmoor Bog. Only one item remains on their impossible quest—the scale of a sea-serpent. Now they must journey to the drowned kingdom, where they will face their deadliest challenge yet. And there they will uncover the truth behind all that has happened... the truth that will change everything.