My dad had a gaming PC around the year 2000-2001. Preloaded on it was a gameplay trailer. It was in first person. Guy enters a dungeon, swings his sword a few times. Groans can be heard from a lower chamber, with a rope leading down to it. Guy climbs down the rope, and at the bottom is this zombie/monster. I remember it being green, but that may or may not be true. He tries to fight it and dies. Does anybody know what I'm talking about?

Could be Stonekeep? But that's from '95.

Definitely not Stonekeep, this was more advanced and fluid. Full 3D.

Sounds like a fantasy-RPG, of which I haven't played many, admittedly. x-P Something like Gothic, Crusaders of M&M, Heretic 2, etc.

This is a total shot in the dark guess and I have no idea if it matches your description or not but the first thing that popped into my mind was Rune https://www.gog.com/game/rune_classic which is from late 2000 so it fits the time scale correctly at least although it's a third person game.

Thief: The Dark Project (1998)
The 3d mission called "Down in the Bonehoard" fits your description.

Wow, I didn't realize that game was that old. I thought the entire series was from 2002 onward! But then I never owned them until a couple years ago here on GOG, although I remember seeing them in stores in the past. If that turns out to be the right game, that's a pretty good award winning call on your part though for sure. :)

I think this is about OP's recollection of a game trailer, which might not even show actual gameplay but a prerendered sequence. Did Thief have any trailer like that?

A trailer where Garrett would try to fight some (green)? zombie doesn't sound very thief-like. Well there was that pre-alpha Thief trailer when the game was still Doom with a bow, with Garrett just running around shooting at everyone with a bow.

So I guess the question mainly is does anyone else remember that specific game trailer, and to which game it belonged.

This looks pretty damn close. I think you got it, and solved a fifteen-year-old mystery.

Hope it is what you were looking for. Thief rocks and is my favorite game of all time, so I recommend you to check it out sometime, you won't regret it :)

I love stealth games and have been interested in that one since I first saw it ages ago. Put it in once to give it a brief test spin but never got around to actually playing it. Everyone speaks quite highly of it though. I'll have to rectify that some time and dig it out of the backlog. Perhaps in a future edition of the Gentleman's Backlog Club(TM). :)

Couldit be Enclave, which is on GoG?

I love stealth games too. Played quite a bit of them, but Thief series is the best of them all! It's "hearing/seeing" system of the AI, beautiful soundtrack, atmosphere and story puts all the stealth games to side which is a shame really. Besides, this game has a huge fan and modding community which gives you the possibility to play this game for years, there are hundreds of fan missions made and many of them are so awesome, that some may say they even surpass the original game :)

Nice catch! I vaguely remember that area. I got my copy of that along with Tomb Raider from buying a Sound Blaster card--it made phenomenal use of that card, btw. Sadly, I never did get past the area after the zombies to free that dude from jail. I will have to play that again.