This is a massive 60m+ leviathan that appears to be the odd convergence of several evolutionary offshoots.
Shovel-like head: The streamlined head of the shadow leviathan helps it slice through the water as it approaches its prey. Beneath its smooth skin is thick head plating that is used as a battering ram to ward off competing Shadow Leviathans during mating season.
Segmented legs: The legs are covered in a thick segmented exoskeleton similar to an arthropod. The leviathan’s legs are used less for propulsion - which is left to its blade-like tail - and more for hunting prey. The thick legs fold open over captured fauna like a cage, while the strong exoskeleton protects the tissue beneath from damage as the leviathan feeds.
Bioluminescent digestive tract: The Shadow Leviathan’s mouth leads directly to its digestive tract, which secretes a highly acidic compound onto captured prey. This bioluminescent cavity attracts light-seeking phytoplankton, which in turn draw in larger fish.
Assessment: Extreme threat - Avoid at all cost.