Doom + Muerta

Why this works

Picking these two cores together puts your team's primary damage into one basket. Doom's ultimate is a primary source of single-target magic damage, meant to disable and burn down a priority hero. When Muerta activates Pierce the Veil, her own powerful attacks are converted entirely to magic damage as well. This creates a redundant damage profile where both of your cores are trying to solve the same problem with the same tool. An enemy team can build a single Pipe of Insight or multiple Glimmer Capes and effectively counter the biggest cooldowns from both Doom and Muerta, severely limiting your kill potential.

This weakness shows up in decisive teamfights. You'll often find both heroes trying to burst the same target, committing two long-cooldown ultimates for one kill. Even if successful, the enemy can often survive through stacked magic resistance while their teammates are free to act. The duo also lacks reliable, hard lockdown to set up for Muerta's damage outside of what Doom might find with Devour. This forces them into these all-or-nothing ultimate commitments that are too easily answered, leaving your team without enough sustained or mixed damage to clean up the rest of a fight.