XCOM 2 is such a good game that I feel strange doing my due diligence to explain it to the unfamiliar—but in case you've not heard of Firaxis' turn-based tactics masterpiece, let me lay the grid out for you.
Getting a 94% from us in our XCOM 2 review back in 2016, this game basically set a new standard for its genre, expanding on the first game—also a banger—with new mechanics and a deliciously oppressive atmosphere that makes every playthrough a nailbiter.
What's really neat about the series is its reliance on permanency. Your squad members can die to an unlucky roll of the dice, and it's supremely easy to enter a tailspin dive where you're trying, desperately, to cobble together enough resources and troops to stop the aliens from flattening you into paste.
It's also $3 (£1.74) on Steam right now for, at the time of writing, 28 hours and 28 minutes, which is a steal of a price for a cup of coffee, let alone one of the best tactic games of the last decade. It also happens to be cheaper than the bus I took to work this morning, and I'm reasonably confident it's a lot more fun than public transport.
It should be noted, though, that the game's marquee DLC, War of the Chosen (which is excellent, and definitely worth a buy) isn't similarly discounted—if you're looking to take advantage, I'd wait on shelling out for it, especially since it was similarly priced at around $2.70 (£2.40) according to SteamDB's price history back in June—and routinely has similar discounts offered.
If you're really strapped for cash, though, the FOMO might not be that bad. The base game has had similar discounts over the past year as well—still, accounting for the considerable gaps in between said offers, it'd be outright moon logic to miss snagging it for considerably less than a Big Mac, the cost of 7 Freddos, or 1/5th the price of a US World of Warcraft subscription.