Can the Eel Come Out to Play?
From the Super Mario Wiki
Can the Eel Come Out to Play? is the second mission of Jolly Roger Bay in Super Mario 64 and its remake, Super Mario 64 DS. Its objective is grab the Power Star attached to Unagi the eel. It can only be completed after obtaining the Power Star of the first mission, Plunder in the Sunken Ship.
The Power Star for this mission is found at the tail end of Unagi the eel, who was freed during the first mission and is now inside of an underwater cave. Unagi must be lured out of the cave by swimming in front of him. After that, the Power Star can be obtained from Unagi's tail, though he starts to quickly swim around. Once the player touches the Star, it moves to the opening below Unagi's cave and the player is able to collect it. If Unagi returns to his cave, the player will have to lure him out again once he pops out.
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