XCOM: Enemy Unknown is a great tactical game from Firaxis games. Its first expansion pack, Enemy Within, added so much amazing content that it made playing the game an entirely new experience. And now there's a mod that's so good it makes the expansion pack feel like a tired retread.

XCOM: Long War is a free mod for XCOM: Enemy Within that maintains the core gameplay but overhauls the entire system to add a depth and breadth of soldier customization, strategic options, and much more. Yes, there's a version for Enemy Unknown too, but it isn't nearly as good. And yes, the mod's been out in beta for more than a year, but a recent major patch added even more content, features, and tweaks.

The core idea behind The Long War is to make the XCOM experience—commanding Earth's last line of defense against an alien invasion—more challenging, more rewarding, and as the name suggests, longer. XCOM is a difficult game to begin with, punishingly so on everything but the easiest setting. Long War takes things to another level entirely. The completion time for training your soldiers, researching projects, building facilities and items—pretty much everything across the board—has been ramped up considerably. XCOM is a war of attrition, and what once took mere (in-game) hours or days now takes weeks or months to complete—while enemy attacks are as frequent as ever.

Chief among these in terms of difficulty is soldier recovery times. In a change from Enemy Within, soldiers experience fatigue for a few days after a battle, even if they survived unscathed. Meanwhile, you should expect any troops with combat wounds to be sidelined for weeks, even months. This means building a deep roster of soldiers to rotate through battles rather than relying on a single six-man wrecking crew.

Long War lets you send up to twelve soldiers into battle, but only after upgrading your dropship. WIRED

Speaking of six-man crews, Long War lets you upgrade your dropship to bring more soldiers into the fray. Later battles will see you commanding up to 12 troops in tactical alien defense. Additionally, the soldier classes have been tweaked and modified to create significantly more role variety and specialization. Each of XCOM's four classes has been split into two specializations, each with its own personalized talent tree. (The heavy class has been replaced with gunner and rocketeer, for example. Sniper has been split into sniper and scout; support into medic and engineer; and assault into assault and infantry.) Class trees now have three abilities instead of two at each level, leading to greater variety in your soldiers' specializations by the time they reach max level.

The Long War also gives you access to psionic powers significantly earlier in the game, and a much larger tree of psionic powers to develop. These mind blasts and psychic takedowns are critical in combating another another great change Long War makes: the alien invaders now also gather resources and research technologies of their own, growing stronger over time if you don't strategize carefully and intervene when necessary.

One of my favorite aspects of Long War is an option called The Not So Long War, which lets you play a campaign with all of *The Long War's *class and item changes but without quite as much of a stretch on everything's completion time. Things still take much longer to complete than vanilla Enemy Within—I could finish EW's campaign in five to 10 sittings, while in that same amount of time I barely made a dent in TNSLW's alien assault—but it's definitely more forgiving than the full Long War experience. Going all in is a significant commitment, but a rewarding challenge if completed—one that truly feels like battling against a mighty invasion force with every scrap of humanity's being.