Chaos Knight counter and synergy picks

SD can use Disruption on both CK and his Phantasm Illusions to create Illusions of his own that inherit Chaos Strike, while CK's own innate Reins of Chaos can be triggered by Disruption. Demonic Cleanse (Aghanim's Shard Upgrade) provides CK with a Basic Dispel and Disseminate increases the value of his Chaos Strike healing as he can now turn it into damage.

Death Pulse and Death Seeker (Aghanim's Shard Upgrade) provide low cooldown AOE healing that can affect CK's Illusions and sustain them in fights, allowing them to deal with one of CK's main counters better - AOE damage. Phantasm's amazing burst pairs well with Reaper's Scythe, quickly dispatching most targets.

In lane the two pair well together as they are melee/ranged - more importantly they both have a level 1 stun with Chaos Bolt and Magic Missile, which allows them to outright win most lanes. Wave of Terror reduces armor just like Reality rift and makes it easier to get to that 0 armor breaking point. CK aims to improve his base damage in order for his Illusions to benefit from it, which scales with Vengeance Aura.

In lane the two pair well together as they are melee/ranged - more importantly Chaos Bolt and Reality rift are amazing at setting up Ink Swell, which allows them to outright win most lanes. Soul Bind allows CK to pull both targets together with Reality Rift and also stun them both with Chaos Bolt.

In lane the two pair well together as they are melee/ranged - more importantly Chaos Bolt and Reality rift are amazing at setting up Poison Touch, which allows them to outright win most lanes. Shadow Wave can both heal CK and his Illusions, while consistently allowing the potential burst of damage it can do to occur as he pulls his targets with Reality Rift. Shallow Grave is amazing at giving CK the opportunity to heal back up with Chaos Strike as it amplifies its heal. Weave reduces armor, thus synergizing with Reality Rift as it makes it easier to get to that 0 armor breaking point.

Nightstalker's lack of AOE damage makes it impossible for him to deal with Phantasm Illusions in a straight up fight, considering they have built in sustain with Chaos Strike. Chaos Bolt has enough of a cast range to outrange Crippling Fear and Reality Rift can be used to pull NS away from his target of focus.

Huskar's lack of AOE damage makes it impossible for him to deal with Phantasm Illusions in a straight up fight. The combination of Phantasm and Chaos Strike can easily catch an already low HP Huskar off guard and burst him in mere seconds as CK can reliably get on top of him with Reality Rift and lock him down with Chaos Bolt.

Viper's lack of AOE damage makes it impossible for him to deal with Phantasm Illusions in a straight up fight. Corrosive Skin provides Magic Resistance, which CK cares little for and the combination of Phantasm and Chaos Strike can easily catch Viper off guard and burst him in mere seconds as CK can reliably get on top of him with Reality Rift and lock him down with Chaos Bolt. Phantasm can dispel Poison Attack and disjoint Viper Strike - even if it doesn't the Illusions do not inherit it's Break and can still Chaos Strike.

Phantasm can disjoint and dispel Ignite. Ogre thrives by locking down and nuking a single target with Fireblast and Unrefinder Fireblast, which even if he manages to land on the real CK his Illusions can still make quick work of Ogre.

Clinkz's lack of AOE damage makes it impossible for him to deal with Phantasm Illusions in a straight up fight. The combination of Phantasm and Chaos Strike can easily catch Clinkz off guard and burst him in mere seconds as CK can reliably get on top of him with Reality Rift and lock him down with Chaos Bolt.

SD can use Disruption on both CK and his Phantasm Illusions to create Illusions of his own that inherit Chaos Strike, while CK's own innate Reins of Chaos can be triggered by Disruption to create additional Illusions for SD. Disruption can also be used as a save to disrupt CK's Chaos Bolt/Reality Rift combo and prevent him from bursting a single target down.

Wall of Replica and Normal Punch (Aghanim's Scepter Upgrade) can create Illusions of CK that inherit Chaos Strike. CK's own innate Reins of Chaos can trigger both on Wall of Replica and Normal Punch, creating additional Illusions. Normal Punch can additionally create Illusions from ANY CK Illusions and not necessarily CK himself.

Blubber can dispel both Chaos Strike and Reality Rift, allowing Tide to counter initiate and stun CK and all of his Illusions with Ravage. Anchor Smash can mitigate most of CK's and his Illusions' damage as most of his total damage output comes from attacking. Dead in the Water (Aghanim's Shard Upgrade) prevents CK from using Reality Rift and his Illusions do reduced damage to it. Kraken Shell provides further damage mitigation against CK's attacks.

All of Medusa's abilities are AOE, making her very effective versus CK and his Illusions. Split Shot allows Dusa to attack CK and any of his Illusions. Reality Rift cannot be used during Gorgon's Grasp. Stone Gaze and Mystic Snake's Aghanim's Scepter upgrade allow Medusa to stun all of CK's Illusions. Mana Shield does not take into account Reality Rift's armor reduction and as much single target burst damage CK can output with his Phantasm, realistically he struggles to go trough all of Dusa's mana pool in a single Chaos Bolt - once Dusa is able to get Stone Gaze of CK is forced to retreat.

CK's total damage output is mostly physical, trough his and his Illusions' attacks, which get greatly mitigated by Axe's infinetely scaling Armor and trigger Counter Helix. Berserker's Call can lock down both CK and his Illusions and Aghanim's Scepter can reduce CK's right-click damage significantly, which is most of his total damage output.