This is a battle of attrition between two high-scaling carries. Karthus wants to land isolated Q blasts from range, while Aphelios relies on his weapon rotations to sustain or burst you. Since Aphelios has zero mobility, your W is incredibly effective at setting up guaranteed damage.

This is a tempo-based matchup that heavily favors you. Aphelios is an immobile marksman with a significantly slower jungle clear than yours, making him vulnerable to invades and isolated Q spam. You win by out-farming him and using R to provide global pressure while he is stuck catching up on gold.