Linux

This might not be what you mean when you say "addictive", but since I've been addicted to it for the last half year or so, I'm gonna suggest it anyway: Morrowind.

While the original came out in 2002 for Windows and later Xbox, there's been a fan remake of the engine which runs on linux (and windows and macos) called OpenMW.

It's an open world role playing game about exploring the island of Vvardenfell, which is a strange and alien place that's easy to lose yourself in. Most of the wildlife is made up of insect- or dinosaur like creatures. There are forests made up of giant mushrooms, and ancient wizard lords who use magic to grow mushrooms into buildings that you have to be able to fly to navigate. It's a world with a rich history, featuring several different religions, cultures and overlapping and competing political structures.

Despite its age, it is to this day a game with a very active modding community which can extend and improve the games mechanics and visuals. It also features what is probably the longest running active modding project, Tamriel rebuilt which seeks to add the rest of the province of Morrowind to the game. It's about half way done and has basically another game worth of content in it at this point.