When The Darkness first manifests, you'll get a glimpse of its full power, as it takes over Jackie's body and mutilates a Mafia hit squad. Once you're back in control, though, you'll have just a few basic powers, although you'll earn back the rest of them as The Darkness powers up over the course of the game. The only catch is that The Darkness powers up by eating human hearts - which, as it turns out, is extremely gruesome (but endlessly satisfying), as one dragon head tears the love muscle out of your dead enemies and then messily gobbles it while staring at you.
In return, The Darkness will protect you from bullets, as well as enable you to send out one of its heads as a tentacle (good for exploring tight spaces, crawling up walls and performing stealth kills), and that can last for a pretty long time as long as you keep it out of the light, which drains its power. Later on, you'll be able to send out a whip-like tendril that can impale and grab enemies, enabling you to effortlessly slam them around or just hurl them into the air for your own amusement. It's also good for grabbing random objects - garbage cans, dumpsters, and even cars at its highest power level - and either stacking them in creative ways or bashing enemies to death with them, which makes for some hilariously creative opportunities for murder.
You'll also eventually earn a pair of Darkness-powered pistols that never run out of ammunition, and - near the end of the game - you'll get the power to open up black-hole voids that suck in everything nearby and kill anything that touches them. These don't always work, but at the very least they'll distract your enemies long enough for you to get a clear shot. Speaking of distractions, you'll also be able to summon stunted, imp-like helpers called Darklings from strategically placed spawn points. Whether you summon a fierce Berserker, a chaingun-toting Gunner, a dynamite-strapped Kamikaze or a self-explanatory Lightkiller, your control over these is limited, and they're useful mainly for buying you time and drawing enemy fire. It's also fun to watch the Berserkers run around in the outfits you find for them, which come with weapons (saws, axes, jackhammers) that make your creepy servants just a little more gruesome than usual.