Quote To opt into the Linux Public Beta Branch:

Right Click on Darkest Dungeon in your Steam Library

Select Properties

Select BETAs tab

Enter Password “1iheartlinux1”

Hit Check Code, should see a Success message

Select Linux_Beta from the Drop Down Box

Close the Properties Window

Install the Game if it isn’t installed already.

If it is already installed restart Steam if the game isn't currently redownloading (restarting should trigger an update to the game)

Article taken from GamingOnLinux.com.

Oh yes! Darkest Dungeon is now an instant buy for many as it has arrived on Linux in Beta form . Lots of people have been waiting for this gothic roguelike turn-based RPG.They are planning for the beta to last around a week, unless of course lots of issues are found.I've requested a Steam key to cover it properly, hopefully they will be able to provide me with one.Darkest Dungeon is a challenging gothic roguelike turn-based RPG about the psychological stresses of adventuring.Recruit, train, and lead a team of flawed heroes through twisted forests, forgotten warrens, ruined crypts, and beyond. You'll battle not only unimaginable foes, but stress, famine, disease, and the ever-encroaching dark. Uncover strange mysteries, and pit the heroes against an array of fearsome monsters with an innovative strategic turn-based combat system.