死敌／致命的危险／严重的威胁
a very serious and dangerous enemy, danger, threat, etc.
The former allies who fought together against the dictator have, in recent years, become mortal enemies.
She was glaring at him, as if he was her mortal enemy.
He soon realized he was in mortal danger.
This set off a chain of events that put all their lives in mortal danger.
It's a story of great courage in the face of mortal peril.
Climate change now poses a mortal threat to the whole nation.
See also: mortal adjective