Crowley, the King of Hell, bid his farewell in season 12 of 'Supernatural', being killed in a face-off with Lucifer after his long journey alongside (and sometimes ahead of) the Winchester brothers starting in season 5.
At least that's what he told Michael Rosenbaum during an appearance on his podcast 'Inside of You'.
"I played the same scene in season 12 five times over five episodes, and I was like "Alright, the gig's up", you know, the gig is up. Everyone was kinda like "Ah, they won't get rid of you, they won't get rid of you", and I'm like: "I'm done, I know I'm done", Sheppard said.
He did not elaborate, however, what scene exactly he was talking about. It's unlikely he meant the episode in which Crowley was killed by Lucifer, but at some point in season 12 Sheppard certainly appeared to have felt like he was being sidelined.
Crowley has become one of fans' favorites after his first appearance in season 5. He evolved from just being a crossroads demon that verified his deals with awkward kisses to the King of Hell himself.
He also got a pretty significant character development, becoming more and more noble over the course of the show, but it seems that at some point Sheppard and the production team just had their creative differences regarding the character's journey.