I started playing Ravenloft without any prior knowledge and without reading up on it, and not too surprisingly, since the design seems pretty open, I explored Barovia and took on all quests I found, including the ghost one, before I had the graveyard key. Because the ghost kept urging me to go to the cemetary, I searched for it and couldn't find it - I had a suspicious but the doors to that area were locked -, so I peeked into a walkthrough and learned that despite the open design the game is so buggy that you should not do things in the order I did and instead avoid entering the ghost's house before you get the graveyard key.

But by now, several hours of exploration and combat and item management and whatnot have passed, and I don't feel like loading a very old save game and more or less start over, so my question is: What exactly should I expect to happen, and will it happen inevitably or only in case I get unlucky? Do I still have a chance of completing the game without repeating the last hours of gameplay since I accepted that quest prematurely?

AFAIK the ghost will kill the possessed character, if you take too long to solve his quest. And you need access to the cemetary to be able to do that. So you can try to do that in time, but I wouldn't delete my old save.

So it's not actually a bug or glitch, but just questionable design? And you mean the character will be completely destroyed, not killed in a way that I could just raise them from the dead again? And then I will get a game over screen or the game won't be beatable anymore with just one PC and three NPC companions?

The game does not end.

The ghost always goes after one of the two starting characters, but any dead character can be replaced by an NPC. Even the two starting characters can be replaced, when they die. The end sequence actually acknowledges the fact that four strangers return to Elturel.

You can try to resurrect the dead character (if it is not an elf), if you still have a cleric in your party. There are many Raise Dead scrolls in the game.

Okay, that doesn't sound so bad. The affected character is a human and I currently have two clerics in the party and at least two Raise Dead scrolls, and if resurrection doesn't work, I also wouldn't mind having more room for NPCs, tbh. ;D

I'm asking because in the walkthrough I checked, there was talk of the quest making the game glitchy and possibly unwinnable, when taken on too early.

At the end of the quest the ghost gives you a key, that is mission critical. If that fails, you're screwed.

I'm assuming if you kicked the dead possessed character from your party, that would happen. If you kept the dead character in your party and revived him shortly before entering the cemetary, it could work.

I seem to have a new problem now; when I enter the temple, the hands of the possessed character are greyed out and the character can't attack anymore (the status display only says "possessed" though, as before, and nothing else). When I step outside, attacking becomes available again, but not inside the temple dungeon. What's up with that?

(The voiceover files of the ghost are all wrong, too, btw. Whenever I go somewhere the ghost doesn't want me to go, the text says one thing, but the voiceovers are saying something different, including the speech for completing the quest.)

I'm beginning to think the game should have been called Ravenloft: The Hero's Possession (by a Vengeful Ghost). ;)

That's why the walkthrough said that you shouldn't get possessed that early. That happens with my save, too. I have a save file where one of my characters has just been possessed and when I go into the temple, my character can't attack either. When I go into the woods west of Barovia my character also is greyed out. So you'll have to make do with only three characters during combat in the temple. The next area then would be the cemetary.

I dig the whole "vengeful ghost wants you to prioritize his quest" thing, but I think it was a pretty dumb idea to make his quest require finishing other quests first and then have him get angry at that. He shouldn't complain as long as you're on the right path that you need to tread in order to get into the cemetary.

I mean, I know what to do and it's still messed up. But without a walkthrough, how would you even know that there is a specific order for how to do things and which things they are in the first place? The only hint you get is that a PC once says something along the lines of "we should look for a big building, possibly with a wall around it and speak to some mayor first". Okay, but how are you supposed to deduct from that you should not look left or right and follow a strict linear route laid out for you but only hinted at quite vaguely?

IIRC, the paladin asks whether you are sure that you are ready for his quest and you can decline and come back later, but the ghost does not.

I think I might try and get cursed real early in my next playthrough. See what happens. In combat actually you only need one character. Give him or her a weapon with a short cooldown, cast haste and only click on that one hand. My fighter mage can attack continuously with the Baal Verzi dagger that way.