Phobos is the larger and innermost of the two moons of the planet Mars, the second being Deimos. It is the scene of the first Doom episode, Knee-Deep in the Dead. "Phobos" is the name of a god in Greek mythology and it can be translated as "panic fear", "flight" or "battlefield rout".
In Doom, Phobos is depicted with Earth-like gravity, a thick atmosphere, and having tall, seemingly vegetation-covered mountains; the sky texture for the episode was derived from a photograph  taken of Yangshuo Cavern in China.
In reality, Phobos is a rock 22 kilometers in diameter with gravity less than a thousandth of that on Earth, and no atmosphere (even if an atmosphere could be generated artificially, the gravity would be insufficient to hold it in place). Phobos' gravity is so weak that a human being could escape it by jumping. In order to be more plausible, Doom 3 moved the plot to Mars.
In Doom 3, Phobos serves as an orbital communications relay station via the Phobos Routing Station Uplink.
In Doom Eternal, Phobos also serves as one of the scenes during the QuakeCon 2018 gameplay reveal, it features an orbital base that is currently under attack by demons, along with a BFG-10000 platform firing periodically towards an unconfirmed location. In the game, the asteroid seems to maintain some level of atmosphere, since mist can be seen extending out into hilly terrain in the distance.