The science-fantasy 4X Endless Legend is free on Steam until May 23, or until they run out of their supply of nebulously free games. That's free to keep, forever, by the way—apparently when they gave it away for free earlier this year they considered it a real success and so they're doing it again.
As part of the sale all of Endless Legend's expansion DLC is on sale from 50-80% off, meaning you can grab the complete edition it including all DLC for just $16.
Endless legend is a Civilization-like 4X game, but the twist is that it's on a science fantasy planet that's sliding into what may well be a permanent ice age. The other twist is that the factions you play as are wildly divergent. One's made of conflict-averse nomads that can just up and move their cities. Another's some smooth-talking dragons that are so good at diplomacy they can just un-declare any war you start with them. A third is made up of weird magical vampires that don't eat food at all.
PC Gamer has been on team Endless Legend for nearly a decade now, since an 89% review back in October 2014, and it remains on our list of the best strategy games on PC. A position that it's likely to retain for a while seeing as nobody has tried to make anything quite like it since.
You can find Endless Legend on Steam.