Ok the original Dark Forces then!

Is it that Rodian crime boss from Jedi Knight 2?
The first mission after you get your lightsaber back IIRC.

Nope and nope :)
The crime boss is neither Jabba nor Reelo.

Dark Forces 2?

The game which I saw didn't have him though, it started out with a different cutscene. Unless you don't mean the literal start but it might happen mid-way through.

KOTOR?

Very close but no :) And it's not the beginning of the game, it's closer to the middle.
Nope, I'm playing an FPS :)

Is it Mysteries of the Sith then?

Yes it is! Level 5, Ka'Pa the Hutt's Palace.

I've never seen it. But then again, there weren't so many games left to choose from.

I stumbled upon the place by chance, killed a few zombies and rat things that attacked me near the entrance and crept underground. Here I am now, wandering through this cursed place time and again. No matter how many rooms I pass, the next one looks exactly the same as the one before. Or is it really just one room that I somehow can't leave?
There was some sparkling thing in that room, I really didn't want to know what it would do to me so I avoided walking through it. Is there really a way out I wonder?

Oh cool, this brings up so many things! At first it fit Oblivion, but then I was - naaah, the rest doesn't fit at all.

Is it a really old game from the 80s? It sounds either like a first person dungeon crawler like Wizardry, or it could be a roguelike like ADOM.

No, the game isn't that old. It has 3d graphics, even though it's not top-notch anymore.
It's a first-person game, yes, but not a dungeon crawler (even though you'll have to go underground more than once).

No idea. Might and Magic? One from the newer series?

No.

walking dead