Undertow is a big white shark owned by the sea-witch, Morgana. A fearsome monster, Undertow was transformed into a small pirancha fish by King Triton's trident until the film's climax, where he is reverted into his normal form by Morgana. He is the secondary antagonist in the Direct-to-video The Little Mermaid II: Return In The Sea, and a minor player in the villains wars, appearing wherever Morgana is involved in any tournament.
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Shark Vs Snake
While Ursula and Morgana were arguing about the control over the seas and the oceans, Messina interfered after she had heard about her old enemy's resurrection, as well as the trident being found in Morgana's hands, and attempted to steal it from her. Seeing the sorceress as a threat, Morgana called Undertow to eat the evil snake. However, just as he was chasing rapidly the snake, Messina jumped from a giant boulder, allowing him to hit a big rock, badly injuring him, much to Morgana's fury.
Disney Villains War
Undertow's transformation into the whale Monstro in Disney Villains War
A New Form
After Hades released the Titans upon Earth, he travelled to Morgana's lair along with the Ice Titan, and tried to usurp her position as the queen of the oceans after she refused her partnership with the god of the Underworld. Although Morgana doubted about Hades and the Titan powers, she made her move by transforming Undertow into a giant whale, and ordered him to deal with the Titan. While the Ice Titan managed to freeze the water below the whale, that didn't stopped Undertow to swim back in the sea and then emerge from the ocean at his full power, driving the Titan away. Unfortunately, Hades took the trident from Morgana's hands and used its powers to kill the giant whale, much to Morgana's horror and dismay.
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Undertow's transformation into a piranha fish