Mortal foe

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

See also: Mortal

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.
What it meant was that they learned to see the US as adversary, rival, or competitor not as a mortal foe.
Lost to England's mortal foe, in those sad, dark days of long ago.
Sleepeth Man, their mortal foe, within the high walls of his mansion.
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".
As the embittered webslinger faces further robot attacks, each deadlier than the last, his spider-sense warns that Jameson himself is behind them, possibly colluding with Electro, Alistaire Smythe, or another of Spider-Man's mortal foes.
Obama's anti-Americanism only invites contempt from both our close friends and our mortal foes.
Summary: <p>Amid rumors that the political furniture is being moved around in Damascus, perhaps the strangest thing is how Syria has organically lodged itself between Iran and Israel, states that are otherwise mortal foes.