I'm gonna keep pushing this title as it's a shitload of fun and highly underrated (even though it's technically not finished. Update 1.0 scheduled before the end of the year, though).
For $15 you're getting Risk of Rain 1 in its entirety as well as Risk of Rain 2 and all future updates it receives.
Basically is a 3D 3rd person shooter rogue-like with the ability to team up with up to 3 friends or randoms as you traverse levels while collecting loot to power up your character (these act as stat upgrades and not weapons like Fortnite).
Definitely a good pickup for some online gaming, and still good fun for solo play!
Risk of Rain 1 also has up to 4-player couch co-op