Captain Tennille's impostor, officially known as Impostor Captain Tennille (偽キャプテン・テニール Nise Kyaputen Tenīru), is a minor antagonist appearing in Stardust Crusaders.
He is an assassin sent by DIO who fights the Joestar Group in the middle of the ocean on their way to Egypt.
Dark Blue Moon is a marine Stand, and as such, his skills are more effective in an aquatic environment. It is probably less effective in a terrestrial environment, but this is unknown.
Dark Blue Moon has a high enough range that it was able to attack Jotaro without the user even being in the water. Its user has stated that it swims faster than any fish in the sea, a trait proven to be true when Dark Blue Moon manages to create a giant whirlpool by simply spinning its webbed feet fast enough.
Its Stand user also has an incredible lung capacity, being able to hold his breath for up to a personal record of six minutes and twelve seconds, which helps the Stand to maintain a battle even if the user is thrown into the water.
Dark Blue Moon's hands are strong and sharp in water, making them effective weapons. Because of their sharpness, they are strong enough to destroy a still running engine propeller and slice a shark in two with ease.
Dark Blue Moon can also use its scales as projectiles, throwing them against the enemy.
Dark Blue Moon is capable of creating barnacles, specifically acorn barnacles, as seen when they attach to Star Platinum soon after making contact with Dark Blue Moon's skin.
The barnacles drain the target of its strength and also allow Dark Blue Moon slight control over the infected body parts, allowing it to drag foes back into the water if they try to escape.