If you happen to be subscribed to Amazon/Twitch Prime, then you can grab the ultra-difficult Titan Souls right now for free. The challenging, boss fight-filled game can be claimed here, but you'll need the Twitch desktop app to download and play it.
It's worth noting that if you'd prefer to have it in your Steam library, or just aren't subscribed to Prime, then you can pick it up at a discounted price on Steam.
In PC Gamer's review, Andi Hamilton scored it an 87/100 and wrote that playing through Titan Souls your first time "is fantastic fun," adding that it's a shame there aren't a few more bosses in its dangerous world.
Previously, the narrative-driven horror game Oxenfree was available to Prime members free of charge. Other games that have been offered with the subscription include The Walking Dead Season 1 and Kentucky Route Zero.
Twitch Prime is a set of bonuses included with every Amazon Prime subscription—once you've subscribed to Amazon Prime, you can take advantage of the benefits. It gives you access to free games, DLC, and other content, in addition to ad-free viewing and the ability to subscribe to one Twitch channel for free—streamers still get the money for your subscription, so no need to worry there. You can learn more about it here.