From seemingly simple beginnings as a noir-ish crime exploit, things quickly get nasty. The power of the Darkness imbues Jackie with a number of unpleasant attributes that are gained as you progress through the game. These often (but not always) take the form of be-tentacled creatures.
First rule of the Darkness (both entity and game): the darker it is, the better it works. It's a factor that makes for an interesting gameplay feature. Fighting on the run can turn particularly nasty when you find you've blundered straight into a well-lit room.
Second rule of the Darkness: some charging is required. You might be able to make your way into a dark spot or shoot out the lights, but if you don't have time to charge the Darkness you might well find yourself pretty dead pretty quickly anyway.
See, the Darkness maybe nasty, primal and, well, dark, but in this game, it’s your best pal. You need it to survive.
The first of the be-tentacle things you encounter are Maw Tentacles. They're particularly ugly things with eyes (which you can see through) and big ol' teeth (all the better, it turns out, for eating the contents of people's chest cavities.) The primary use for the Maw Tentacles is reconnaissance and puzzle solving. Their ability to snake like a giant black phallus around corners and through vents can come in immensely handy, as can their functionality in eviscerating folk. Your Darkness level will also benefit inordinately from using them to devour the hearts of the fallen. Charming stuff but, according to the BBFC, heart devouring is only worth an 18 certificate.
The Maw Tentacles, alas, can be a bit clumsy on the control front. More specifically, getting them to go through a narrow opening at a right angle (as you slither up a wall and reach a vent, for instance) is fiddly. It can take up to half a minute to do this, and with your Darkness level rapidly draining that can be frustrating to say the least.
The other Darkness abilities, however, fair better. Dark Tentacles can be used for shifting heavy objects in your path, Darkness Pistols essentially shoot Darkness, and the ability to create black holes is extremely handy when you're facing hordes of oncoming foes.
Also at your disposal as you progress are the Darklings. These little blighters can be summoned from portals scattered throughout the maps; they perform a range of functions. Stuck in front of a door that only opens from the other side? A Beserker might open it for you. Need a bit of a hand in a fire-fight? A Gunner's your chap. Too much artificial light getting you down? A Lightkiller can sort that for you. Just want to watch one of them blow themself up? Pull a Kamikaze out of the floor.
The only complaint I have about the various Darkness powers is that from time to time a relatively simple puzzle will crop up, but with all the different abilities at Jackie's disposal it can take longer than it ought to work it out. Otherwise, the dark beings serve to keep things interesting without unbalancing the game play.