Leviathans are purple variants of the legendary Leviathan, battled in Story of Nightfall in Skylanders: Ring of Heroes. Though they were small in number and mildly dangerous around Fogshadow Tower before, a giant one once nearly destroyed the town in Nightfall's backstory, and a school of the fish, empowered by a strange crystal powder, razed its shores later on.
Purple leviathans can use their powerful bites and size to attack with their body or nearby water, and can gain health by biting their enemies. However, their ancient hides are easily breached and cause Diminish on the beast when attacked.
Nightfall once found a large egg while exploring the Poison Sea around her hometown. When it hatched the largest Leviathan anyone had ever seen, she evacuated Fogshadow Tower and distracted it to help her kind, earning Master Eon's attention.
In Story of Nightfall, an entire school of Leviathans was empowered by a mysterious crystalline powder, becoming enraged and powerful. Though Nightfall was sent to investigate villains escaping from the Poison Sea, she found herself struggling to fight a single newly hatched monster, and she and the other Dreadwalkers were forced to leave the town behind.
With the help of Spitfire and Stormblade, she returned to the town to salvage what was left from the rampaging beasts at the shore, and they managed to breach through the group and defeat their leader. With its defeat, the other monsters swam back to the sea.
It is unknown if the fishes' strange coloration was caused by the powder or if Poison Sea Leviathans have specialized camouflage.
Poison Sea Leviathans appear to be smaller than their colossal blue cousin, as Stormblade suggests attacking either one's head or rump in quick succession during their assault.