Bloodseeker + Venomancer

Why this works

Feeling the slow tick of Venomancer's poison is already a problem, but it gets much worse when you see a Bloodseeker charging at you from the fog. Venomancer is great at applying damage-over-time to multiple heroes with his spells, which constantly activates Bloodseeker's Thirst. This gives Bloodseeker vision and a huge movement speed advantage to chase down anyone trying to disengage. A big Poison Nova can effectively light up the entire enemy team for Bloodseeker, making it difficult for low-health heroes to retreat or juke into the trees. The synergy gives a lot of map-wide pressure.

The main issue is that this combination lacks hard lockdown and reliable burst damage. In lane, they can harass well but struggle to finish a kill against heroes with an escape. Later on, both Venomancer and Bloodseeker can be quite fragile. A coordinated team can often turn and burst down the Bloodseeker before he gets to his target, or they can simply ignore the slow damage from Venomancer and focus on the bigger threats. The duo relies on enemies being weak and scattered, and it falls apart if the other team can force a direct, head-on fight.