At the beginning of your end step, put a Decay counter on Fog of the Damned. Then if there are no creatures on the battlefield, sacrifice Fog of the Damned and extract 4. (Exile the top four cards of target opponent’s library. If you don’t control a Nightmare token, create a */1 black Nightmare creature token with ‘This creature gets +1/+0 for each creature card your opponents own in exile.’)

Creatures get -1/-1 for each Decay counter on Fog of the Damned.