Batman/The Flash: The Button

The road to DOOMSDAY CLOCK begins here. During the unforgettable events of DC UNIVERSE: REBIRTH, Batman found a mystery he can't even begin to solve a strange bloodstained smiley-face button embedded in the Batcave wall. All analysis suggests the button is not of this universe...so where did it come from? And who left it here? These are questions only the Flash can help answer.