Demon Arm is a power of The Darkness. This power spawns a tentacle from Jackie to impale and lift targets. It can be used to remove, lift, interact with, and toss things such as debris, furniture, and vehicles, as well as break street lamps to create Darkness.
Jackie Estacado obtains this power after consuming the heart of one of the german undead soldiers in The Otherworld.
It is a very efficient ability, being able to kill enemies instantly, remove any movable objects (this may include cars at higher levels) and destroying light sources, while costing almost no Dark Energy.
The Demon Arm has a certain level of auto-aim, automatically impaling the nearest thing to the crosshairs available at that moment.
The Demon Arm did not make a return in The Darkness II, now being replaced by the right serpent head being used in slashing motions, though lacking the insta-kill capacity, it now stuns enemies and can be used with more flexibility.