Darkness (暗黒剣, Ankokuken?), also known as Dark Blade, is a command in the Final Fantasy series, associated with the Dark Knight job. It allows the Dark Knight to use various special attacks, effects ranging from absorbing HP from enemies to dealing dark-elemental damage. It should not be confused with the Dark Knight's Souleater ability.
Darkside (or Darkness in the japanese translation) is part of the Dark Knight skillset and is learned after completing Ishgardian Justice quest at Level 30. The skill increases damage dealt by 15% at a cost of slowly draining own MP. The Dark Knight will also not receive any MP regeneration effect from outside sources as long as Darkside is activated. Should the Dark Knight's MP drops to zero, Darkside will be deactivated.
In Final Fantasy Tactics, Darkness is the action ability of the Dark Knight job. It includes draining abilities as well as powerful damaging attacks based on the user's HP. Darkness abilities require an equipped sword to be performed.
The Dark Knight uses Dark Blade. As the Dark Knight's job level increases, more Dark Blade attacks are learned. The skillset contains various damaging attacks that can also harm the user to offset their power.