overlord (n.)

late 12c., "a feudal superior, ruler possessing the fealty of other rulers," from over- + lord (n.). In English history, especially a king of one of the Anglo-Saxon realms who held pre-eminence or authority over some of the other kings and chiefs. The word was chosen in 1943 as the Allied code-word for the invasion of western Europe that resulted in D-Day.

 In the perspective of 'Overlord,' that one huge hazardous offensive operation on which, it seemed, the fate of the world depended, smaller adventures receded to infinitesimal importance. [Evelyn Waugh, "Unconditional Surrender"]