Chaos Knight + Crystal Maiden

Why this works

A single Frostbite is often a death sentence. Crystal Maiden provides the reliable root that lets Chaos Knight walk up and guarantee his own stun, followed by a Reality Rift to keep the target close. This chain of lockdown is usually more than enough time for him to secure a kill. The real engine of this partnership, though, is Arcane Aura. It solves Chaos Knight's chronic mana problems, allowing him to use his spells aggressively for kills and farming from the very start of the game. He can play at a pace he normally can't afford.

This duo enables a highly aggressive playstyle right out of the gate. They can hunt for kills in lane constantly, and this pressure often snowballs into a mid-game where they roam together to delete heroes. In teamfights, enemies are caught in a difficult position. They have to deal with Crystal Maiden's disruptive Freezing Field, which often gives Chaos Knight and his illusions the space they need to isolate and destroy a core. The main way to fight this is to burst the Maiden before she can act, because if she gets her spells off, her Chaos Knight is already in a winning position.