https://www.gamespot.com/articles/free-shadow-of-war-dlc-adds-more-endgame-content-b/1100-6454180/

Some gameplay structure spoilers about Middle-earth: Shadow of War follow below, though no specific plot details are discussed.

Middle-earth: Shadow of War developer Monolith is beefing up the Lord of the Rings game's endgame content. At the end of the main game, you are attacked by orcs who are attempting to reclaim the fortresses you took earlier in the story as part of a system called Shadow Wars. Presently, this battle comes to an end, but the game's first expansion--The Slaughter Tribe--will make these encounters endless.