http://www.pcgamer.com/middle-earth-shadow-of-war-will-get-infinite-shadow-wars-in-a-coming-update/

War never changes, as the saying goes, and soon it will never end, either—at least, not in Middle-earth: Shadow of War. Monolith revealed in a recent livesteam that "infinite Shadow Wars," which will provide endless fort defense at the end of the game, will be released alongside the Slaughter Tribe Nemesis expansion. 

Shadow Wars is basically an end-game sequence in which the forces of Mordor launch counter-attacks against fortresses held by the good guys. (That's you.) It sounds like a terrible grind to get the game's "real" ending, but apparently a lot of players quite like it, and they want the option to carry it on indefinitely. Monolith said during the stream that the option is "coming very quickly," although a specific date wasn't revealed.