Anti-Mage + Legion Commander

Why this works

Controlling the map becomes difficult for the enemy team because they're forced to choose between two bad options. Grouping up to survive Legion Commander's ganks and Duel attempts opens up the map for Anti-Mage to farm freely. Spreading out to catch Anti-Mage, however, leaves heroes vulnerable to being picked off by Legion Commander. When they do fight together, a Duel provides the long lockdown needed for Anti-Mage to drain a target's mana. A follow-up Mana Void can help secure the Duel damage and the kill.

During the laning stage, these two offer very little to each other, as Legion Commander plays an active role while Anti-Mage focuses on his own economy. The synergy appears in the mid-game. Even if Anti-Mage is split-pushing, the threat of him using Blink to join a fight and turn a key Duel is always there. This forces the enemy to play more cautiously or over-commit resources to what should be a simple gank, creating breathing room for your team across the map.