Primeval Spinosaurus

An ancient river-leviathan of a spinosaur, its sail scarred and barnacled, that rules a whole watershed and drags entire boats under without breaking stride. Nothing that drinks from its river does so twice unwatched. It has ruled its drowned kingdom for centuries, grown vast and patient, and the river-folk leave offerings at the water's edge rather than risk its notice - it takes fish, beast, boat, or war-band with the same unhurried certainty.

Leviathan Bite: piercing, melee 15 ft, +4 to hit, 5d8+7 piercing; a held target is dragged into deep water and drowned (3d8 each round) on a failed Dex Quick save, cooldown 2 rounds. Breaching Lunge: it erupts from the water to strike a creature on shore or deck for 4d8+7 and knock it prone, cooldown 3 rounds.

Amphibious: master of deep water, ambushing from below like a drifting island of weed. Leviathan Bulk: immune to being held, moved, or knocked prone; resists nonmagical damage. Senses: Night Vision (120 ft), Living Sense (feels the living, 120 ft).