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Non-canon warning: This article or section contains information that may not be considered an official part of the Metroid series in the overall storyline by Nintendo.
Rastodon (ラストドン Rasutodon) (also known as the Sea Demon) was a massive underwater anglerfish-like beast seen in chapter 6 of Metroid EX. Rastodon, whose body produces concentrated sulfuric acid, was dropped into the water of the ocean world and quickly began to poison it. This was part of a trap to lure Samus Aran into the water, to her death. She would die quickly in her Power Suit, without the Varia upgrade, now possessed by Rastodon's master, Defen.
Samus learns of a "Sea Demon" from the townspeople and assumes this is Defen. On the beach, Defen drops her comrades Joey and Diesel into the poisonous water, which is the final part of Defen's plan to kill Samus, as it provokes her into diving in after them. She quickly begins to suffer from the effects of the acid on her Power Suit, in addition to the attacks from Defen and Rastodon and so Samus falls, weakened, to the ocean floor. However, motivated to fight for Joey's sake, she discovers an inactive volcano and triggers an eruption, which saves her and kills Rastodon and Defen.
Rastodon's full body.
Non-canon warning: Non-canonical information ends here.