Bloodseeker + Phantom Assassin

Why this works

Getting bursted down by a Phantom Assassin is bad enough, but having her on a team with Bloodseeker adds another layer of pressure. When PA lands a big crit and her target tries to escape, Bloodseeker's Thirst activates from anywhere on the map. This gives him vision and a massive speed boost to chase, making it very difficult to get away safely. Even if the target survives the initial jump, they're now marked for death. The slow from Stifling Dagger also provides a decent setup for Bloodseeker to land a Blood Rite, silencing them and often sealing the kill.

This combination mostly comes online in the mid-game, turning ganks into more reliable kills. Phantom Assassin doesn't need Bloodseeker to function, but his presence punishes any enemy who manages to survive her initial assault. The opposing team has to play around this by grouping up and using defensive items to prevent PA from getting a target low in the first place. If they can weather the initial burst from Phantom Strike, they can often turn the fight. The synergy excels at cleaning up after a fight starts or punishing split-pushers, but it's less effective against coordinated team pushes.