Combo skill is available for all classes, not only Dark Knight. To do combo player must use skills in right order, the correct order is: First skill and after any other from next skill list. Combo will be activated after 3 skills sequence, and after last third skill usage.
Dark Wizard, Soul Master, Grand Master, Soul Wizard combo
First Skill: Energy Ball
Next skill: Aqua beam, Inferno
Dark Knight, Blade Knight, Blade Master, Dragon Knight combo
First skill: Falling, Lunge, Uppercut, Cyclone, Slash
Next skill: Twisting Slash, Rage-full Blow, De