Aspidochelone

The Aspidochelone is a sea beast so immense that sailors mistake its shell or back for an island. Sand, weeds, and even small trees collect upon it while the creature rests near the surface. Fires, anchors, and gathering crews eventually wake it; then the false island descends and the sea closes over everything built there. The encounter is an evacuation problem before it becomes a battle.

Island Submerges: every creature standing on the Aspidochelone makes a Quick save or is pulled 40 ft underwater and takes 4d10 bludgeoning damage.

Read the Living Shore: noticing rhythmic movement, shell seams, or the absence of nesting birds allows evacuation before submergence. Removing every anchor prevents the creature from attacking for 1 round as it turns away.