Absence is a Wolfenstein 3D mod by Havoc that was released for DOS on December 23, 2008 and later reissued for Wolf4SDL on April 24, 2011. It features a dark sci-fi story about genetic and robotic experimentations run amuck in a U.S. army research base called the "Sigma Complex". Featuring 18 levels, it contains puzzle elements, an experience system, story-driven diary entries and new weapons and enemies.