A truly fun FPS about stopping unholy forces and the people assisting them from trying to take over our home (I am unclear if it's Earth or this dimension but it's certainly one of them) with the use of a variety of guns.

This is one of the first, if not the first, game to to be in a truly 3D environment. It's also among the first to have music-music in a game, the soundtrack of which is made by Nine Inch Nails (with plenty of atmosphere segments). This game is also the reason we have games like Half Life and Team Fortress today, because of it's engine.
This game has flashing lights, but they can be changed in the settings. It also mutes when you alt-tab.
You have multiple different campaigns on offer, and while they CAN be played in any order, I recommend playing in the order that is displayed, as they will only get harder and harder the further down you go, regardless of the difficulty you choose. You can also play the campaigns with other people, though if you're achievement hunting, you must go in alone.
Speaking of achievements, there seems to be something a bit wonky going on with some of them, as at least for me a few did not pop even though I fulfilled their requirements.
As for multiplayer, the modes I personally played were capture the flag and a free-for-all death match with some friends, and both were fun, but very under populated, and very much meant for more people than we had. If you can get a group of 6 or more people together though, I highly recommend giving it a try.
ALSO, if you have any plans to stream this game to Twitch or Youtube, it would be wise to mute the game's music, as it will get your vod muted.

Now go, and save us from the evil forces of Quake!

trent reznor

quack