Evil Ground is a two-part story released under The Gunslinger line of comics, which actually takes place before The Little Sisters of Eluria. The first issue was released April 3, 2013, and the second June 5, 2013. They are collected in Last Shots.
Robin Furth wrote Evil Ground as a story-within-a story-within-a story, just as Stephen King had done with The Wind Through the Keyhole. The story starts with Roland Deschain heading towards the Desatoya Mountains. He falls asleep and dreams of his old ka-tet's adventure, when they too fell asleep and dreamt collectivily of a different reality.
See Also Chapter Summaries: