They are the Shadow of Mordor games, based in the LotR world. You play a protagonist who’s story takes place “behind the scenes” of the series. They go on sale or marked as free at least once or twice a year. I personally haven’t found time to play through them but always want to!

Kill a mini boss of an orc horde. Orc horde gains a new boss that evolves around how you killed thier predecessor. Rinse and repeat and the game gets harder and harder while you also git gud.