Spawn of Dagon

Spawn of Dagon by Henry Kuttner is a sword-and-sorcery story set in the world of Atlantis, where the hero Elak of Atlantis must confront a plot by the followers of Dagon to sink the island. These followers, known as the "children of Dagon", are using black magic and earthquakes to destabilize Atlantis and ultimately submerge it. Elak, with the help of a mage named Zend, must stop them and protect Atlantis from being destroyed.