It's the same developer who created Relic Hunters Zero, a free to play and open source action game. While that doesn't currently support Linux officially (they're still working out some issues), it does have a Linux build on Steam that isn't advertised. It seems they're going all-in for Linux support with Relic Hunters Legend!
About the game:
Fast. Fluid. Fun. Gather up to 4 friends online and go on a galactic adventure! Embark on a touching tale of friendship and time-travel, find awesome loot, customize your Hunters with deep RPG systems, and experience the next-generation of top-down shooter combat with gorgeous 2.5D visuals! The Relic Hunters universe is also expanding into comics and animated series!
Since Relic Hunters is a persistent online game, our goal is to tell a deep, non-linear, character-driven story across many years, just like a TV series. We already have a rough script for the first Season, and it will flesh out the Universe, present characters new and old, and reach a satisfying conclusion, while still leaving many opportunities open for the next Seasons to pick up the story.
The developer says it will go into Early Access after crowdfunding, but once it's complete and finished it will become free to play. They claim it will have no pay to win or pay-wall of any kind, everything will be possible to earn in the game directly. They claim they want to "help fix Free-to-Play games" so they say in their "Free-to-Play Manifesto".
I have to admit, the style looks fantastic and the gameplay does look pretty damn fun. This could be a really great game to play with friends. They claim it will have advanced enemy AI, solo and co-op play with multiple characters, a deep RPG system and more. They've even put up a web-comic here, which has multiple episodes.