You had taken the mark of Cain. In doing so, you created a deep connection with Amara, The Darkness. When you removed the mark, killing death, Amara was set free.

You were in the Impala with Sam and Dean one second, watching as black fog raced towards you, and the next you were standing in a field, the fog all around, minus a bubble around you. Amara stepped out of the fog, into the clearing, in front of you.