There are a few things you absolutely need to know about Leviathan, Mass Effect 3's first story based DLC post-extended ending, which was released a little more than a month ago.
First, if you've finished the game, you'll be able to jump into Leviathan using the save that Mass Effect 3 created when you completed it. This is set at the point-of-no-return just prior to the start of the final act.
Second, this is the first of several planned releases of story-based downloadable content for Mass Effect 3. Leviathan is scheduled to release at some point this fall.
Third, it has the sort of things you'd want from Mass Effect DLC after the precedent set by Mass Effect 2's excellent series of releases. Leviathan has everything it's supposed to, mind. It has a new mystery to explore involving a research project probing suspected ancient Reaper technology. It looks like there's the requisite red herrings and plot-twist moments, and it even takes the Mass Effect series to an environment it's literally never been to before.
I hesitate to say more, because plot is sort of the point of playing the DLC - I would hate to spoil any more of Leviathan than the premise. From the bit that I played, you can expect an extended wave-based survival section and another solo-Shepard exploration segment, which came to define much of Mass Effect 2's DLC as well.
Here's the problem: as someone who considers Mass Effect the experience which defined this console generation, as someone who loved Mass Effect 3 and its ending, I can't seem to bring myself to care about any new DLC content for Mass Effect 3.
The problem isn't multiplayer downloadable content - that makes sense. Mass Effect 3's multiplayer has found surprising legs, and new maps and weapons are status quo in this situation.
But Leviathan is different. Story-focused DLC releases worked to Mass Effect 2's advantage - some of the greatest moments from ME2 happened over the course of the Overlord and Shadow Broker DLC releases, allowing BioWare to add meaningful character development that they didn't have the time or resources to include in the campaign. But Mass Effect 2 also had an open ending. It ended with possibilities, with potential. It ended with a protagonist that players were promised would continue with the release of the next game.
Those promises were kept, of course, but BioWare can't make those same promises with Mass Effect 3. Much of Mass Effect 3's narrative success hinged on the catharsis of a mission completed, a story finished. Regardless of anyone's complaints or lack thereof regarding Mass Effect 3's ending, I haven't talked to anyone who thought their Shepard's story was left open to further adventures.
Even the extended ending DLC requires a suspension of that closure, setting the game back in time to the beginning of the final act so that players can replay and experience minute changes here and there before a more dramatic expansion of exposition later on. But that works somewhat because its leads to the same conclusion that was there before - it's finished.
I feel like I've finished the Mass Effect series for the time being
Meanwhile, Leviathan is new story content that pretends the game hasn't ended yet, arguably the worst possible way narrative downloadable content can be appended to a story-focused title. Granted, Leviathan is available to players early in new or existing, unfinished playthroughs of Mass Effect 3. For those players, Leviathan is more - but it's more of a substantial game that those new or in-progress players haven't finished yet. Downloadable content exists primarily to prolong the value proposition for early adopters, for players who have completed or made their way through most of a game.
So we're back where we started. I've finished Mass Effect 3, and I feel like I've finished the Mass Effect series for the time being. I'm not sure how BioWare and EA can hook players like me back into a game that so firmly closes the door on its main character, but they'll need to figure it out. There are more campaign DLC releases scheduled for later this year and 2013, so it's a problem I hope they have a plan to solve. They've only got a few months to go before Leviathan comes to Xbox Live and PSN this fall.