Darkness is a black fog, that was originally printed all the way back in Legends. A fog is a spell that prevents all combat damage this turn. It gets it’s name from the card Fog which has been a part of the game since Alpha. Just like Withering Boon it is a card that is completely unexpected. This is because it is not something that black can traditionally do.
Darkness has been received an errata it now reads “Prevent all combat damage that would be dealt this turn”. The oracle text can be found here.
Why should you run it?
Just like the original Fog, Darkness only costs a single mana but this time it is Black. If you have ever played magic against someone like me, you know just how devastating the combat step can be. Furthermore combat is an important step for most Boros and Gruul decks, and essential for infect decks. When you are building a Commander deck you want to make sure that you have interaction or answers. Additionally you want to make sure that you can answer a range of different problems, including combat.
Darkness can buy you a turn if your opponent has an overwhelming board state. Or it can cause a massive blowout to your opponent if they have used pumpspells or other combat tricks. It can also be used as a political tool, you can save an opponent in exchange for a later favour.
The downsides to running Darkness are minimal, and you only need to leave a single mana open to cast it. This means that it can go in just about any deck that is running black. For example: K’rrik, Son of Yawgmoth; Yawgmoth, Thran Physician or Kaalia of the Vast. It is also good in Thantis, the Warweaver decks as you can get the +1/+1 counters and avoid the combat damage.