One element is the lyrics. If the lyrics are about dark topics like death, ghosts, or curses, it can be scary. For example, lyrics that describe a spooky encounter with a supernatural being. Another is the tone of the rap. A slow, deep, and menacing tone can add to the scariness. Also, the setting where the rap takes place. If it's in an abandoned place like an old factory or a haunted house, it gives a creepy vibe.
Well, here's one. In a small town, there was a rap battle. A young rapper named Jay took part. He rapped some really bold lyrics. But after the battle, he started having nightmares. In his dreams, he was in a dark room and a shadowy figure was rapping at him, criticizing his skills. Every night the dream got worse, and the figure got closer. One day, he decided to stop rapping, and the nightmares finally ceased.
Sure, the Noragami manga is completed. It was a great ride with an ending that wrapped up the plot nicely. If you haven't read it yet, it's worth giving a go.
Noragami is about a minor god named Yato. He wants to have a grand shrine and a lot of followers. He meets a girl named Hiyori who can partly leave her body due to an accident. They get involved in various supernatural events and battles against other spirits and gods. Yato also has a regalia named Yukine who he has to deal with as Yukine has his own growth and problems.