Last generation when released and first round of testing this generation, it was banned to Ubers due to it's sheer Speed, powerful Special Attack, access to Nasty Plot, accurate signature move Dark Void which puts the foe to sleep, and its signature ability, Bad Dreams, that depletes a good chunk that sleeping foe's HP.
Special Attack: 135
Special Defence: 90
Now why would I make a thread about the powerhouse? Well, there was the recent event. I recently got a shiny one in a trade (it's most likely hacked but there's no definitive proof). The main reason is Smogon's reconsideration for it's place in the Dream World tier (which will eventually merge with OU). Here's a link for their discussion and the link to their Pokemon Online Dream World OU tournament where Darkrai is legal.
Anyways, discuss and good luck sleeping tonight.