The village's skepticism had turned to a hushed anticipation as Kai set out once more, the memory keepers' stone secured within his weathered coat. The sea called to him, its waves a roadmap to the last guardian.

Kai's journey was a solitary one, the ocean vast and unforgiving. Yet, with each passing mile, the stone's pulse grew stronger, its glow a beacon in the relentless blue. It was as if the ocean itself recognized its chosen champion, parting its waves in silent reverence.

The song that had once been a distant whisper was now a clear call, resonating through the water and guiding Kai to the depths where the last guardian waited. It was a place untouched by sunlight, where the sea's secrets were kept, and its history was written.

There, in the darkness, Kai found the last guardian. It was a whale unlike any other, its size dwarfing the boat that carried him. Its skin was etched with the stories of the sea, and its eyes held the wisdom of ages.

The guardian's song was a lament, a mournful melody that spoke of loneliness and the heavy burden of being the last of its kind. Yet, within its notes, there was also hope—a plea for the return of the harmony that had once been.

Kai approached the guardian, and the stone reacted, its light intertwining with the whale's song. The visions returned, more vivid than before, showing Kai the path that lay ahead. He saw the world as it could be, a place where humans and whales shared the stewardship of the ocean, where the songs of the deep were heard and heeded.

The guardian entrusted Kai with its song, a melody that held the power to awaken the slumbering life within the sea. It was a gift and a responsibility, a legacy that Kai now carried.

As the whale submerged, leaving Kai alone on the surface, the weight of his task settled upon him. He was the bridge between two worlds, the voice that would unite them in a symphony of hope.

With the guardian's song etched into his heart, Kai turned his boat towards the shore. The journey back was one of contemplation, of planning the steps he would take to fulfill the prophecy and restore the ocean's balance.

The last guardian's song was not just a call to the creatures of the sea; it was a call to humanity. And Kai would be its herald.