Source
From the preview:
Playing the Enemies of the Imperium mode adds a sort of doomsday clock to the Relic experience, and the game feels more like a race than ever. With a Nemesis in play, the defenders of humanity can no longer afford to be complacent. Once a Nemesis reaches 25 infamy, the game is over, the Nemesis player wins, and a grievous blow is dealt to the Imperium of Man.