Downwell will be added first, on August 1, with Space Hulk: Tactics coming on August 8. Confusingly, the announcement also lists Slay the Spire as a future Xbox Game Pass for PC game, but it's already there—what's new is that it's being added to the console Game Pass.
Xbox Game Pass for PC is currently $5 per month at its introductory price, and offers access to 100-something games through the new Windows 10 Xbox app. One downside is that games which are moddable on Steam probably aren't when purchased through Microsoft. Slay the Spire really is better with Steam Workshop access, so if you like it, you might consider picking it up a la carte at some point.
And, obviously, Game Pass being a subscription service means that if you cancel your membership, you lose access to the games. If you don't mind having fleeting experiences with them, though, you can get more than your money's worth in a couple months and then cancel. According to the site, Microsoft is still offering the first month for $1.