It must be an unknown foe; if so, the man might have realized Harry's presence.

Sharnas twisted under her touch, struggling against some unknown foe.

He wanted to seize the unknown foe alive, for nobody would be helped by his death.

Behind crime lay an unknown foe, who fostered evil and kept it on the move.

He had merely been able to receive the order which insisted that the law be stirred to immediate action against an unknown foe.

He was determined to identify the face of his unknown foe.

As he turns to leave, he is struck down in one blow by an unknown foe.

There was no defence against this unknown and all-powerful foe.

Within moments the crew scrambled to their stations to combat this unknown foe.

Its ruddy light showed Bob, coming to his feet, about to return the attack of his unknown foe.