Reincarnated as an NPC and Decided to Work Hard to Upgrade

Reincarnated into the game world I played, I became a passerby character without even a name. Monsters are raging, dragons cover the sky, and humans are lingering inside the high walls - this should be the worst hell. Until I discovered that the "effort" here will be quantified into experience points, and there will even be a horn echoing in my head when leveling up. "Where is hell here? This is heaven!" I abandoned the plot, ignored the heroine, refused to socialize, and devoted my entire life to a never-ending leveling carnival.