Unreal is a first person shooter published in 1998 (aka "the greatest PC gaming year ever"), by a company who at that point was famous for making a rabbit platformer with guns.
I still remember the days, when I first saw the menu intro. The caste fly-by, and the iconic music, with the beautiful graphics. While many great games came out in that year, arguably Unreal was the best looking one, almost as it looked… unreal. The water effects, the transparency effects, everything looked like something from another era. Back then we didn't know that this will be one of the primary graphics engine which helps moving forward the gaming industry.
Other than the graphics, Unreal had an incredible music, which changed according to what the player were doing. If we just exploring, there were nice relaxing, calming music, but as soon as we meet the monsters, the dynamic music kicked in.
The game also had one of the best AI for its time, rivaling with Half-Life. Enemies moving around, shooting, rolling, etc. It's far from the days of the default, "they coming at you until either of you are dead" AI.
The whole cherry on the cake is the environment. We are prisoners on a ship, and we just crashed on a planet. The first few minutes, we are escaping from a ship while everything is shaking, and we saw a glimpse of something other worldly.
Then we get outside of the ship and we see a beautiful green planet. We hear the birds, we see the small animal that's just jumping in front of us. This all has shown us naturally, not with a cutscene. We can look around and go discover this amazing place. From this moment we are going against the monsters, but also meet the native ones who can help us to get some weapons and upgrades for these weapons.
How to play it on modern machines and on Linux?
First of all - as always - buy and download it from GOG. Use Wine or Lutris to install it.
Second, go to the OldUnreal page, where you can find the modern patches for it. Unpack it to your Unreal Gold folder, than run the exe installer inside that Wine prefix.
Warning: save games are not compatible with the patched version. So before you start the game, install the patch and then start playing. You will know if the install is successful, because the menu image will change to OldUnreal.
After installing the patch, you can change the graphics renderer, as there is a new DIrectx 8 & 9, plus an improved OpenGL ones are available.
The above page has tons of mods and maps for the game.