Aether-Shark

Ecology
The Aether-Shark is a mutated aquatic predator found in the flooded sewers of Steamfort and the magically volatile depths of the Broad Swells. Their skin is translucent, revealing glowing Aether-conduits that pulse with power. They have evolved to feed on the magical runoff of civilization, making them exceptionally aggressive toward spellcasters.
Combat Mechanics
Aether-Sharks are incredibly fast in the water. They possess a "Lightning Breath" attack (a 15-foot cone) and their bites deal additional force damage. They have "Aether-Sense," allowing them to detect the casting of spells within 100 feet. When a spell is cast nearby, the shark can use its reaction to move up to half its swimming speed toward the caster.
Loot
The most valuable part of the creature is its "Aether-Infused Fin." When ground into a powder, it can be used to create Potions of Speed or oils that grant a temporary swimming speed to the user.