Luna + Mars

Why this works

Getting trapped inside Arena of Blood is usually a death sentence, but against this pair, it can feel surprisingly survivable. The problem lies in how their ultimates interact. Mars's Arena creates a solid wall that blocks line of sight. If Luna is outside the Arena when she casts Eclipse, her beams won't hit the targets Mars has so conveniently trapped. For the combo to work as intended, Luna has to be inside the Arena with the enemy team, which is an incredibly dangerous place for a carry, especially before she has her core defensive items.

This dynamic forces a difficult choice in teamfights. Either Mars uses his ultimate for control and Luna holds Eclipse, or Luna has to risk her life for a chance at a big ultimate. The result is often that one of their most powerful spells is used sub-optimally. While Mars can still create space and set up stuns with Spear of Mars, the lack of a clean, devastating ultimate combination means this pairing doesn't provide the overwhelming teamfight threat it might seem to on paper.