XCOM 2 just got its biggest DLC expansion yet, Alien Hunters. It adds three new "ruler" aliens, which function as bosses who, unlike other XCOM 2 enemies, can escape and appear in multiple battles. I was surprised to find that it's kinda like the Nemesis system from Shadow of Mordor. Alien Hunters focuses on three ruler aliens who can show up across your whole campaign. If you begin a fresh campaign with the DLC enabled, you'll get a story mission that introduces them. It's pretty decent. If you pile the DLC atop a pre-existing mid-to-late-game campaign, they will just start ambushing you. Maybe they will show up during a low-stakes mission, maybe they will stomp on your lunch (and/or skull) when you've got two turns to complete an objective that will shave precious cubes off the Avatar Project.
You take on one at a time, and they looooooove being real dicks about pretty much everything. They fundamentally break XCOM 2's rules in more ways than one. Not only can they live to fight another day and follow you between encounters like a lonesome Mordor orc chief, they also ignore turn order. They get to make a move each time any of your squad members do anything. Old strategies go out the window as your central priority shifts to, "Stop them from moving so goddamn much." It's an interesting shift, definitely one of the biggest I've experienced in XCOM for quite a while. The bosses are tough, but not too tough once you figure out how to use new, stun-based items like the Frost Bomb.
On the upside, the DLC adds a new layer to the gut-wrenching tales of love and loss that play out behind your eye curtains in XCOM 2. It might not be as sophisticated as Shadow of Mordor (aliens certainly don't trash talk you or staple their heads back on after you slice them off), but it put me in a similar state of mind. These aliens and me, we had history. No matter the battlefield, when we met, it mattered.
On the downside, there's not a whole lot of meat on this one's bones, and once you've killed the alien rulers and turned their skin into battle pyjamas, it kinda wrecks the balance of the rest of your campaign. You get some damn powerful items, and once the trifecta of rulers goes from unholy to mouldy, there's hardly anything else on their level.
Personally, I'm looking forward to seeing what modders do now that Firaxis has introduced the concept of persistent alien nemeses to XCOM 2. Already, there's a mod that lets rulers regain HP after they have fled from a battle. Who knows what the future holds: More types of rulers? Different behaviour patterns based on past battles? Hatred for specific squad members? I'm not sure how much of that is possible within XCOM 2's framework, but I'm hopeful.
In the meantime, here's the Saga of my first encounter with an alien ruler, The Snake King, in video form: