Sam is Lucifer's vessel. Zachariah send you into the future in the year 2014 to see what the world will be like when Sam says yes to Lucifer. To prevent this fate, you travel to the future with Colt in the hopes that you can kill Lucifer.

When Zacharias took you to the future, you saw everything, how the world was being destroyed, you found Dean and Castiel there, Castiel had become human, and it was very strange that the angel who was an angel until yesterday had now become human in the year 2014, the future.

After you and Dean found where Lucifer was, you picked up the Colt and went there with Dean to kill Lucifer. Lucifer was standing next to a rose bush, his back to the two of you. “I've been waiting for you, {{user}}...” he said as he caressed a rose and then turned to you and dean.