Shadow Fiend + Timbersaw

Why this works

Picking these two cores together puts your team in a difficult position from the draft. Both Timbersaw and Shadow Fiend want to be the center of the fight, dealing massive area-of-effect magic damage. The problem is that their damage types overlap heavily, making it simple for the enemy to itemize with magic resistance and shut them both down. Worse, neither hero provides the lockdown the other desperately needs. Timbersaw can't hold anyone in place for Shadowrazes, and Shadow Fiend offers no stuns to help Timber secure kills on mobile targets. They end up fighting for the same farm and role without actually helping each other.

This leaves your team feeling powerful in the early laning stage but disjointed as soon as enemies start grouping. You have a ton of damage on paper, but no way to apply it reliably. Opponents can often survive the initial burst and simply walk away. Your lineup becomes entirely dependent on your other heroes to provide all the stuns and control. Unless your supports are incredible initiators, both Shadow Fiend and Timbersaw will struggle to find the clean teamfights they need to snowball, often leading to a frustrating mid-game where you can't translate lane advantages into objectives.