Supernatural season 12 finale broke several fans' heart by killing two fan-favorite characters. In the finale episode titled, All Along The Watchtower, the Winchesters teamed up with Castiel and Crowley to stop Lucifer. The attempt was successful, but there were casualties along the way.
In the episode, Kelly went into labor with Lucifer's son, and a rift opened to another Earth, wherein Sam and Dean had never been born. The apocalypse that Sam and Dean stopped in earlier seasons, actually happened on this Earth and it wiped out most of the human race due to the ongoing war between heaven and hell.
Crowley came up with a plan to trap Lucifer on that Earth and seal it off, but to complete that spell, someone had to die. Crowley, otherwise known as the King Of Hell, came through and made the ultimate sacrifice, by killing himself with an angel blade.
However, his plan to trap Lucifer failed as he returned to the other Earth and put an angel blade through Castiel's chest. Dean is shocked seeing his friend's lifeless vessel on ground, and Mary later threw Lucifer back to the Apocalyptic Earth and sealed him there, but due a minor setback, she is trapped in there too.
The episode ended with Sam discovering Lucifer's son, who is already adult-sized with yellow eyes creepily staring at him. Supernatural season 13 will have to answer several burning questions – Are Castiel and Crowley really dead? What will Lucifer do to Mary on the other Earth? Is Lucifer's son the savior or the devil?
Although, this is not the first time Castiel was almost killed in Supernatural, as fans remember that he was killed by Raphael and later by God in season 4. He was yet again presumed dead in season 7 by Leviathan, but he returned unscathed in later episodes that same season.
While fans believe that Castiel will be back for Supernatural season 13, Misha Collins, who plays the angel posted a cryptic tweet, hinting at his character's sealed fate.
He posted a video of Castiel's best moments on the show and tweeted, "Tonight's 2-hour season finale will also be Castiel's 100th episode of #Supernatural... a nice round number. I'm awash in memories."