The phrase "mortal enemies" means two people or groups that hate each other so much that they would do anything to harm or destroy each other. Full definition
The New York Post, mortal enemy of ACORN, an organization that, unfortunately, hasn't existed in years, still loves to drag out that dreaded acronym in hopes of scaring its dumber readers with the thought that somewhere poor, probably brown people might be stealing all our democracies. (thealbanyproject.kinja.com)
Though the Soviet Union and the U.S. had become mortal enemies by the 1960's, the two adversaries are ordered to work together to prevent an international criminal organization centered in East Berlin from acquiring the knowledge to make an atomic bomb, which they expect to sell to any country with the money and thereby increase world tension. (shockya.com)
There's something undeniably appealing about teaming up with mortal enemies toward a common goal. (tomsguide.com)