Ash and Ichor

"The Fates are rather cruel, aren't they? Childhood wasn't enough; they had to take my mortality too." Percy wiped the soot off his face, and the god in front of him prayed it was only the shifting of the shadows that made his eyes gleam with rage. "I have two months until the rest of my blood turns the same shade."

Or, Percy was the one wearing Luke's shoes in the Lightning Thief. Three years later, he gets brought back to camp with ichor in his veins.

Or, Tartarus is not kind to those that lived in its depth.