The Yellow King is an Early Access massively-multiplayer online world with a dark horror theme and it's now available on Linux.
You can see some early footage from the developer below:
While in-development, it doesn't currently seem to have large and active player-base which is a shame as it looks like it could be a lot of fun when played with others. However, the overall user review score gives it quite a positive picture. Perhaps now it's on more platforms, they might start to see extra players trickle in. In the announcement, they mentioned they're quite a small team so they will continue to fix issues as they come up.
You can find The Yellow King on Steam.