Before BioWare had even released Mass Effect 2, they promised a bevy of quality downloadable content that would serve to expand upon the universe, and the story. The newest piece of DLC, Overlord, is the biggest and best of these downloadable adventures thus far. Clocking it at about two and a half hours, Overlord isn't a significant piece of DLC, but it does feature some of the most well-designed, and varied missions the series has seen, and features an excellent, haunting side story.
Overlord takes place entirely on one rather open planet. Players will go between duking it out with enemies in indoor laboratories, and exploring with the Hammerhead in a pretty expansive outdoor environment.
The best thing about Overlord is how it toys with the formula players have gotten used to in Mass Effect 2. In the game proper, players go between shooting, and talking, and while it works rather well, the game begged for more variation, and Overlord pack features some attempts to spice things up (even if some ideas do fall flat). As mentioned earlier, the hub world in Overlord is the first environment in the game to feature both on-foot, and Hammerhead sequences, showing that planetary exploration can still work even without the Mako. But there's plenty more deviations to the formula including a sequence of platforming across rivers of lava in the Hammerhead, and a rather odd puzzle. These additions aren't groundbreaking by any means, but they're fun changes to the usual formula.
The story is the star of the show, however. The basic setup for the plot is that a virus that has been terrorizing a research base is trying to go off-planet to wreak havoc on the rest of the galaxy, and, of course, it's up to Shepard and his team to stop it, all the while dealing with a sketchy scientist. This story drives you through the first portion of the DLC, but clues as to what's really going on pop up along the way, and there is a grand reveal moment at the end. Once the full scope of the plot is understood, it makes for a great sci-fi story, it's weird, mysterious, and haunting, and it ranks as one of the coolest side-stories in the Mass Effect universe.
It goes without saying the Overlord is merely a new adventure, it doesn't tie in with the overall Mass Effect storyline, and it doesn't present any significant changes to the formula. That being said, Overlord has some unique gameplay moments (even if some of them are questionable), and an awesome story that make it worth the price of admission.