Shadow Fiend + Clockwerk

Why this works

Picking an initiator for Shadow Fiend means finding someone who can hold a target still for Shadowraze and Requiem of Souls. On paper, Clockwerk seems to do this. A good Hookshot into Cogs provides a long duration of lockdown on a single hero. The problem is that the Cogs themselves are a physical barrier. For Shadow Fiend to land his spells, he either has to be right next to the Cogs—dangerously close to the fight for a squishy hero—or the Clockwerk has to knock the enemy out of the Cogs, which can mess up aiming. It creates an awkward positioning puzzle that many other initiators don't.

This friction means the combo often feels clumsy. An early gank might work out, but as the game progresses into full teamfights, the lack of clean synergy shows. A Clockwerk will trap a key target, but Shadow Fiend may not have a safe angle to unload his damage without exposing himself. A perfect Requiem of Souls on a cogged target requires the Shadow Fiend to essentially be inside the fight, which is usually a bad trade. While they don't actively work against each other, the heroes lack the smooth setup that allows Shadow Fiend to really take over a fight.