The phrase "an ancient enemy" is correct and usable in written English.
It can be used to refer to a long-standing adversary or rival, often in a historical or metaphorical context.
Example: "Throughout history, the two nations have been locked in conflict, each viewing the other as an ancient enemy."
Alternatives: "a long-standing foe" or "a time-honored adversary."
Her literary perambulation encompasses ovine love life (the block of wax crayon attached to rams in order to denote which ewes have been tupped is called a "ruddle"), bracken ("an ancient enemy… colonises land at hideous speed") and Thomas Hardy's self-designed house in Dorchester ("staggeringly ugly").
Battle against an "ancient enemy" in a First World War setting, with beefy visuals and a choice of tactics.
Unfortunately, just as the Traveler's arrival heralded a new Golden Age for mankind, an "ancient enemy" of the mysterious being arrived; "a powerful darkness that found us here on Earth and nearly destroyed us".
Saddam's contemptuous last words on the gallows about "Persians" - along with Americans and "spies" - struck a familiar chord in a Sunni world that is increasingly obsessed by an ancient enemy.
In Greece (outside Thessaly), Philip could have had no illusions about his own unpopularity, except among those of the well-to-do who were attracted by his court and his patronage; some cities also (especially the neighbours of Sparta) were glad to lean on Macedonia for support against an ancient enemy.
Thor forms an alliance with treacherous Loki to save Earth and the Nine Realms from an ancient enemy that predates the universe itself.
The player takes the role of one of the mysterious Guardians as they fight against The Darkness - an encroaching ancient enemy.
In one discussion in a test version of the Web site, users debated the question, "Is your hook 'high concept?' " (A sample from a writer who signed in as K. S. Brown: "A vampire is kidnapped and tortured, and the man her secret government sends to rescue her is a weredragon, the ancient enemy of all vampires").
Ironically, Austria existed from 1815 to 1914 in a symbiotic relationship with her ancient enemy, the Ottoman Empire.
After two generations of fighting against inflation, why be worried if the victory looks just a bit too complete, if the ancient enemy is so cowed as to no longer strain against the chains in which it is bound?
The series will pick up with Ash in the present, where he's apparently still a stock boy dragged back into battle against his ancient enemy.
