Short Answer 
Ryuk, a Shinigami in “Death Note,” drops a supernatural notebook that allows its user, Light Yagami, to kill anyone by writing their name. As Light becomes increasingly obsessive and tyrannical in his quest for a crime-free world, Ryuk’s view of him shifts, and in the final act, Ryuk writes Light’s name in the Death Note, ensuring his demise and ending the story that fascinated him.
Step 1: Ryuk’s Role and the Death Note
In the story “Death Note“, Ryuk is a Shinigami who drops a supernatural notebook into the human world out of boredom. This notebook allows its user to kill anyone by writing their name in it. Ryuk remains neutral and is simply an observer, intrigued by how a human, Light Yagami, uses this powerful tool, making choices that reflect the moral complexities of life and death.
Step 2: Light’s Downward Spiral
As Light Yagami becomes increasingly obsessed with his new power, he aims to create a crime-free world where he reigns as a god-like figure. However, his methods grow more erratic and violent, leading to numerous conflicts with characters, including the detective L and others trying to stop him. Ultimately, his desire for control and power contributes to a change in Ryuk’s perception of Light, viewing him as no longer interesting or worthy.
Step 3: The Final Confrontation
In the climax, Light attempts to kill Ryuk using a piece of the Death Note to escape his fate. However, Ryuk intervenes by writing Light’s name in his own Death Note, which is aligned with the rules of the Death Note that guarantee death upon writing a name. This act is consistent with Ryuk’s unpredictable nature and self-interest, ensuring his own survival while ending Light’s reign, marking a fitting conclusion to the tale Ryuk has been fascinated by.