Whenever Grotag Night-Runner deals combat damage to a player, exile the top card of your library. You may play that card this turn.
If you don’t play the exiled card, it remains exiled.
You must follow the normal timing permissions and restrictions for the exiled card. If it’s a land, you can’t play it unless you have land plays available.
You’ll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.