Mortal foe

an inveterate, desperate, or implacable enemy; a foe bent on one's destruction.

See also: Mortal

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in classic literature ?
``Admit him,'' said Cedric, ``be he who or what he may; a night like that which roars without, compels even wild animals to herd with tame, and to seek the protection of man, their mortal foe, rather than perish by the elements.
But all of my own sex - especially one impostor, three or four years my elder, with a red whisker, on which he established an amount of presumption not to be endured - were my mortal foes.
'In fact, this war was the quite-predictable fruit of a centuries-old bitter enmity between the two countries set against the backdrop of an increasingly strong alliance between Vietnam and what was now China's mortal foe in the Soviet Union.
Bush, who had branded the North part of an "axis of evil." Now, as president of South Korea, Moon is keen not to repeat past failures as he stakes his own political career on brokering a deal between the unpredictable leaders of the US, his nation's protector, and North Korea, long its mortal foe.
The Syrian regime was its mortal foe to the north so no Israeli leader wishes Assad well.
Played by Matthew McConaughey, he is Elba's mortal foe. The film combines elements of several of the books.
What it meant was that they learned to see the US as adversary, rival, or competitor not as a mortal foe. Iran has not yet cured itself of that disease and Obama's weakness may make it even more difficult for that cure to be applied.
That summer of 1980 in Washington, DC, Ninoy decided to speak out against his mortal foe, Ferdinand Marcos.
A-Zahar, a long-time mortal foe of Israel - he told BBC in 2003, "I'm telling you frankly, the attitude of Islam is not to accept a foreign state in this area" - blamed Israel for the lack of progress in the talks on the release of Shalit.
This time she's his mortal foe, the Wicked Queen in the same production of Snow White.
Chapter four is about the New Illiteracy, called "The mortal foe of my children".
Lost to England's mortal foe, in those sad, dark days of long ago.