The quote "The Underworld had no mercy. It only had justice" from Rick Riordan's book "The Blood of Olympus" reflects a stark reality of the afterlife in his mythological universe. It suggests that the Underworld operates on strict principles, prioritizing a form of justice over compassion or forgiveness. This portrays the realm as a place where past deeds are weighed, and individuals face the consequences of their actions without leniency.

This perspective emphasizes accountability and moral order, highlighting the themes of justice that are prevalent in many mythologies. By stating that mercy is absent, Riordan reinforces the idea that the Underworld serves as a final arbiter, ensuring that all souls receive their due retribution. This concept adds depth to the narrative, illustrating the complexity of life, death, and morality in the story.