Night Stalker + Nyx Assassin

Why this works

Hunting a lone hero at night with this duo is a death sentence. Night Stalker can initiate with Dark Ascension, reducing enemy vision and applying a long silence with Crippling Fear so the target can't escape or fight back. This gives Nyx Assassin a free setup for his Impale and the burst damage from Vendetta. The chain of disables is long enough to kill most heroes before they can even get a spell off. Both heroes excel at moving around the map unseen to find these pickoffs, making them a natural ganking pair.

This constant threat forces the enemy to change their entire game plan. From the first night cycle on, farming the outer lanes becomes a huge risk, and supports can't safely place wards alone. The enemy team has to group up much earlier than they'd like, which slows down their farm and map control. Their best bet is to stay together as five and force fights in well-warded areas. If Night Stalker and Nyx can keep the map dark and find isolated targets, they'll create a mid-game advantage that's very hard to overcome.