A landmark shooter just turned 20. No, it’s not a Doom clone. It’s Epic’s Unreal! Released on May 22nd, 1998, Unreal brought a cutting-edge 3D graphic engine to the masses (by comparison, id Software’s Quake was only a mere eight months old when Unreal was released). Sure, it wasn’t as fun as Quake, but man, the game looked miles above what id had done.

In fact, without Unreal, we wouldn’t have many of our modern horror games (at least not looking the way they do), as the Unreal Engine has been the root of many of our games, including Clive Barker’s Undying, Alice: Madness Returns, Deadlight, the Outlast series, Friday the 13th: The Game and the list goes on.

To celebrate, Epic has the game available for free for the next 24 hours on both Steam and GOG.com. In addition, GOG.com also has the rest of the series on sale for a very reasonable 80% discount. That one is available until May 29th. Really, you have no excuse to not get in on video game history with this one.