Spotlight on Chaos Knight (6.85)

Chaos Knight is a powerful carry, with both mid-game and late-game strengths.  Unfortunately, he's not terribly strong in the early laning phase (no spammable nuke, no escape skill, not ranged), and can be countered by several currently-popular heroes, including Ember Spirit.  He also lacks the ability to flash-farm and get the lead that he needs to dominate.

However, he's been given small buffs lately, namely that his Chaos Strike passive now gives armor reduction when it crits, and a few patches ago, his Phantom Steed ability also has a 50% chance to give an extra illusion.

This doesn't change Chaos Knight all that much, however - he still needs to be itemized the same way, for the same reasons, and his skill build is also unlikely to change much.

Spell Combo:

Chaos Knight's spell combo is:

Phantom Steed + Rift + Chaos Bolt + Rift.  

Done correctly, this should net him a kill in the early and mid game.  The problem is that CK doesn't have enough mana from stat gain to cast this combo!  This often leads to people delaying skilling up his ultimate until he can be itemized for additional mana.
 
At L1 Ult = 125 + 50 + 140 + 50 = 365 mana (CK barely has enough mana if he has Treads on Int)
At L2 Ult = 200 + 50 + 140 + 50 = 440 mana ( "  " )
At L3 Ult = 275 + 50 + 140 + 50 = 515 mana ( "  " )

Itemization:

Chaos Knight suffers from a small mana pool and terrible Int growth.  This means that if he wants to cast his spell combo he needs some Int from items.  He ALSO needs teammates with Arcane Boots or Mekanism, if he wants any sustain - otherwise he's back to the fountain after each fight.

Early Items for consideration include:

• Wand - some stats, but not a lot; wand charges accumulate usually AFTER the combo.
• Bottle - replenishable regen, but no stats or mana pool gain.
• Null Talisman - cheap early mana, but no upgrade path.
• Power Treads - switch to Int before casting Phantom Steed, then switch to Str.
• Drums - solid midgame item, also benefits illusions (stats).

Of all these items, I believe Power Treads should be your first go-to, although it's more expensive than Wand, Null, or Bottle.  Because of that, you should buy a Robe of the Magi after brown Boots, instead of a Belt of Strength - this will allow you to go for that early kill.

Big Items:

Items that CK really wants include:

• Heart
• AC
• Manta
• Skadi
• (Boots of Travel)

Situational Items:

• Armlet (really good early damage, but needs a "health recovery" item)
• BKB (if lots of magic damage or stuns)
• S&Y (good only if finished early - can be disassembled for Manta + Halberd) 
• HoD (good for early lifesteal if no Vlad's on team, Alpha Wolf companion)

Laning:

Because CK needs a good start, you should lane with a support that combos well with him, such as AA, CM, SS, Lion, Lina, Disruptor or Jakiro.  I feel that using defensive supports with him (such as Dazzle) are a mistake, because it reduces the chance that CK can get kills and the early boost in farm that he needs to dominate other carries and snowball.

Pro Scene

Roaming Support:
CK saw a little bit of play in October 2015 as a roaming support.  This choice is a high-risk, high-reward play that didn't pay off in the game I watched (link TBD), and leaves CK without any contribution in the mid and late game, and no catch-up mechanism.  I think the issue is that CK really needs at least level 2 before he even starts to roam, and level 7 before he becomes truly dangerous.