Click on the spiral on the level selection screen after completing level twelve to play the Death Blow level.
Note: This procedure involves editing a game file; create a backup copy of the file before proceeding. Use a text editor to edit the "shellcfg.h" file in the "dark\shell\" directory. Change the "#define BTN_MISSION_COEFFICIENT 150" line to "#define BTN_MISSION_COEFFICIENT 157". The new "Cheat" icon in the lower left corner of the screen will allow any mission to be selected on the dial.