Xonotic

Xonotic is a free and open-source[1] first-person shooter video game. It was developed as a fork of Nexuiz, following controversy surrounding the game's development. The game runs on a heavily modified version of the Quake engine known as DarkPlaces engine. Gameplay is inspired by Unreal Tournament and Quake, but with various unique elements. https://en.wikipedia.org/wiki/Xonotic