Magnus + Treant Protector

Why this works

Picking these two heroes together feels like you're committing to one big teamfight plan. The obvious combo is for Magnus to use Reverse Polarity to group enemies for a perfect Overgrowth from Treant Protector. The problem is that the abilities have a lot of overlap. Reverse Polarity provides a long stun, which makes the root from Overgrowth mostly redundant on those same targets. You get the damage, but you're layering two huge control spells without getting the full value of both. It's a massive cooldown investment for an effect that doesn't stack as well as it might seem on paper.

In practice, the success of the duo hinges entirely on that one ultimate interaction. During the laning stage, Treant Protector can certainly help Magnus stay healthy with Living Armor, which is a clear positive. Later on, though, the game plan is very rigid. If Magnus fails to land a big Reverse Polarity, the whole strategy fizzles out, and Treant has to use his ultimate defensively or opportunistically. It creates a situation where if the combo works, you were probably already winning the fight; if it fails, the heroes don't offer each other much else to fall back on.