While Foothills of Mordor is in the staging area, 'when revealed' effects cannot be canceled.

Forced: After Foothills of Mordor becomes the active location, raise each player's threat by 2.

...there lay dark thickets, and a tumbled land of rocky ghylls and crags, behind which the long grim slopes of the Ephel Dúath clambered up.
–The Return of the King