Lucille is a baseball bat covered in barbed wire first encountered in Issue 100 of Image Comics' The Walking Dead. It is used by Negan to kill both human and walkers alike. After Negan's fall in Issue 126, Dwight became the new possessor of Lucille.
In Issue 123, Negan brings out a new and improved Lucille that is covered in walker guts, so that when hit, survivors will get infected and die. Lucille is also the tool used to kill the beloved character Glenn Rhee