Doctor Who The Power of Kroll by Terrance Dicks

This narrative follows the Doctor and Romana as they land on the swampy moon of Delta Three to retrieve the fifth segment of the Key to Time. They become entangled in a conflict between a group of Earth colonists running a protein refinery and the Swampies, the planetoid's native green-skinned inhabitants who worship a legendary creature called Kroll. The refinery’s director, Thawn, seeks to provoke a war to eliminate the natives, while the Swampies prepare to sacrifice Romana to appease their god. Amidst a violent storm, a colossal, mutated giant squid—the real Kroll—emerges from the depths to devastate both the refinery and the native settlement. The Doctor ultimately discovers that the beast’s massive size was caused by its ingestion of the Time segment, which acted as its sacred "Symbol of Power." By using his Tracer to reclaim the segment, the Doctor causes the monster to dissipate into harmless smaller squids, thereby ending the immediate threat and the colonists' occupation.