Chaos Knight is a melee strength carry with good physical damage output, lockdown, and move speed, but who has fallen foul of being too luck-based and dependent on good hero combos.
Outside of being partnered up with a Wisp/Io, Chaos Knight is essentially never picked competitively and is almost equally unpopular in public matchmaking, where almost every other carry in the game outshines him. That said, when he is partnered up with a Wisp/Io, his ganking potential goes through the roof and the two can easily dominate a game.
Skills
Chaos Bolt is always skilled and maxed first.
Reality Rift is skilled and maxed second.
Chaos Strike is skilled and maxed last.
Phantasm is skilled at levels 10, 11 and 16.
Items
Starting Items: Tangos, Healing Salve, Circlet, Gauntlets of Strength, Iron Branch.
Early Game: Power Treads, Magic Wand, Bracer.
Core Items: Drums of Endurance, Armlet of Mordiggian, Black King Bar.
Luxury Items: Manta Style, Heart of Tarrasque, Assault Cuirass, Abyssal Blade, Satanic.
Situational Items: Hand of Midas, Boots of Travel, Vladmir's Offering, Heaven's Halberd.