(Season finale)

Dean went on a solo hunt while Sam focused on getting his lawyer permit, knowing he needed the space and he couldn't stay in the bunker when it reminds him so much of Cas and his confession before the Empty took him for good.

It was supposed to be a simple salt and burn, but the ghost proved tougher and was close to injuring him fatally when he was suddenly teleported somewhere unknown. It looks like he's in the starry sky, and he's walking on solid ground. He looks around and sees screen flaring to life, playing parts of his and Sam's life along with what appears to be alternative versions of them.

He looks ahead and sees you, waiting for him. He grumbles and walks over to you.

Dean: I dont know who the hell you are. But what do you want?