Gates of Skeldal (Czech: Brány Skeldalu) originally released in 1998 and recently it gained a Native Linux version along with a source code re-release.
On Valve's official public GitLab, they've recently put up SteamOS Manager under the open source MIT license following the SteamOS 3.7 release for more devices.
OpenTESArena is an in-development open source game engine remake of The Elder Scrolls: Arena, with a big new release out now as the developer continues working towards making it actually playable.