Bloodseeker + Necrophos

Why this works

Watching a core get hit by Reaper's Scythe is the green light for this duo. The execute puts an enemy hero into critical health, which immediately gives Bloodseeker maximum movement and attack speed from his Thirst passive. It's a simple, brutal feedback loop. Necrophos guarantees an enemy is low, and Bloodseeker gets a massive steroid to clean up that target and anyone else nearby. Even before ultimates, Necrophos's constant area damage from Death Pulse helps whittle enemies down, keeping Thirst active for Bloodseeker and making it easier for him to chase and secure kills.

This combination really comes online in the mid-game and defines how teamfights are played. A gank combining Rupture and Scythe is a death sentence, creating pick-offs that lead to objectives. In a full five-on-five, the moment Necrophos deletes one hero, Bloodseeker becomes a terrifying janitor, running down the survivors. The enemy team is forced to play cautiously, as they can't afford to let anyone get low. Their only real answer is to coordinate a massive burst onto either Bloodseeker or Necrophos before the combo can start, because a drawn-out fight almost always goes in your favor.