A request has been made concerning a mysterious legendary pokémon of the sea.
The Request
Check on the blackboard in Professor Laventon's office in Galaxy Hall in Jubilife Village to accept this request. Professor Laventon thinks that the mysterious pokémon that he saw has gone to the Cobalt Coastlands. Warden Iscan tells you that the Cobalt Coastlands used to be called the East Sea, and a princely pokémon lived in the waters there. He says that there was a mysterious tome called "The Sea's Legend" that spoke of this pokémon.
Find a Clue
The only clue for this mystery is in Pokémon Brilliant Diamond and Shining Pearl.
A book called "The Sea's Legend" can be found in the library in Canalave City.
The text of the book is as follows:
Once upon a time in the East Sea, there was a Pokémon known as the prince. A brave human asked Pokémon living in the sea to let them see the prince. Mantyke, Buizel, and a Qwilfish with huge spikes acknowledged the human's bravery and joined them. Together, they set off in a boat over the sunset-streaked sea, sailing through the ocean gate stretched over the waves. News of this reached the ears of the prince, who went to meet the brave little party at the Seaside Hollow.
The Solution to the Mystery
Catch a Mantyke, Buizel, and Overqwil (can be caught during The Trial of Lake Valor or evolved from Qwilfish by having Qwilfish use Barb Barrage 20 times with Strong Style). Put them into your party. Sleep at the Coastlands Camp in the Cobalt Coastlands until evening, then sail on Basculegion between the two horns that stick up out of the water. If you did it correctly, you will hear the sound of a boulder shifting in the distance. Go to Seaside Hollow (north of Veilstone Cape) and enter the cave. You will encounter a Manaphy and three Phione and can try to catch them.
Report to Professor Laventon
After you battle Manaphy and Phione, talk to Professor Laventon.
Reward
You get 3 Comet Shards.