Way back in Supernatural season 6, the angel Michael tried to continue his war against Lucifer, and ended up locked in Lucifer's Cage. We've heard tell of Michael since then (mostly that he'd gone insane in the Cage), but we haven't seen him.
That will change in Supernatural season 13, as Sam and Dean have to deal with an alternate world in which there are no Winchesters, and Heaven and Hell are locked in war. And in this alternate timeline, Michael is still very much alive and active - as you can see in the first-look photo down below, courtesy of EW!
This version of Michael (who isn't possessing the form of Adam Milligan, obviously) will be played by actor Christian Keyes. As showrunner Andrew Dabb describes, it will be a very different version of the character:
“The Michael of apocalypse world is the victor. He’s a very different character than the one we met even in season 5. He’s Genghis Khan. He’s been through the wars. He’s conquered the world.”
Dabb also teased a season 13 story arc that will "Apocalypse World" and the real world colliding, so The Winchesters will once again have to face Michael - only it will be a far more formidable version.
Supernatural returns for season 13 on Thursday, Oct. 12 at 8 p.m. ET on The CW.