A hidden lagoon in the jungles of Brazil; holds a secret - a monstrous creature half man and half fish. The creature runs amuck on an isolated rubber plantation, a plantation owned by Japanese expatriots. Some of these still have strong ties to their homeland and a message is sent. This is a creature that could cause havoc to the enemies of the Rising Sun.
The creature is captured and subdued using explosive charges and torches. A U-Boat is despatched from the Third Reich to bring a horrific cargo to Florida.
The Man-Fish is a 5 HD creature. Its alligator like skin absorbs four points of damage. It has 1d10 usage die to represent its limited endurance out of water. The creature's level for making or avoiding attacks on it is considered to be half the highest number on its usage die, so it starts at 5 (1d10), then becomes 4 (1d8), 3 (1d6) etc to a minimum of 2 (1d4). The usage die is rolled every minute of normal activity, every moment of combat.
The creature is extremely cunning and stealthy. characters may be called on for a Wisdom or Intelligence save to detect a sneak attack by it. If they fail the roll the creature's claws do double damage.
Fighting the creature under water is not advised. Characters foolish enough to fight it in its element roll with disadvantage to hit, block or do damage. Even going near the water is dangerous as the creature can still make a sneak attack and the saves to detect it are rolled with disadvantage.
The Man-Fish is incredibly strong, able to overturn a car or fling a grown man like a doll. It is also able to make 10-20' leaps out of water of any depth over its head.
Attacks with flame or electricity allow a character to make a Charisma save to drive the creature away or at least keep it at bay.
It is unknown if the monster is a unique relic or a small population of them exists deep in the jungle. Their cognitive ability is also unknown. They may be trainable as underwater shock troops. for that matter the creature itself might be a victim of cruel men and more docile than initially reported.