A variation of the Force Cloak, the Darkness ability is not strictly a Dark Side power, although currently it remains unique to the Darksider known as Sivter. It was developed by him as a way to hide himself without having to resort to using the power that a Force Cloak requires.
With the understanding that darkness is the absence of light, but light isn't the absence of darkness, the user of this ability can create areas of darkness in even the most well-lit locations. These areas of pure darkness only last for as long as the user can maintain the power and will quickly vanish once they’re done. The brighter the light, the harder it will be for the user to create or maintain the artificial darkness.
Although it doesn’t require the same amount of power and control that a Force Cloak does, the Darkness ability is not for beginners. Learning how to use it can be a good step towards learning to create the cloak though.
In addition to hiding himself, Sivter has found another use for the Darkness ability: spreading fear and intimidation. Seeing all trace of light being sucked from a room as he enters tends to prey on the primal fears of all kinds of sentients.