So, I've been playing Serpent Isle and I'm about to head to the Furnace where the Serpent Staff resides. There's a well-known bug with Xenka that will prevent her from talking about sacrifice, which is crucial to finishing the game. The bug is related to the Serpent Staff, so: what exactly do you have to do to prevent that bug from appearing later in the game? Thanks in advance for any help.

I found this - "At 71S 52E, you'll find a ruined powder mill and a ghost that will play a major role in the endgame. He's Ssithnos, the last Great Chaos Hierophant, which won't mean much to you now, but will later. Anyway, he was a major
power among the Ophidians.

Note: Save your game before talking to Ssithnos, then restore afterward. There's something buggy hereabouts. Much later in the game, you'll find another way to talk to Ssithnos, in another location. If you've already talked to him here, you may hit a bug with the prophetess Xenka that will keep you from finishing a major part of the game."

Just don't talk to the powder mill ghosts at all. It's a spoiler at best and a gamebreaker at worst.

As for the serpent staff, I never had a bug with it, so I don't know. Could just be an Exult thing.

The bug with the serpent staff is trigger based. The best way to make sure you avoid it is to walk right up to the staff before you pick it up to ensure that the trigger goes off. If you're using teleport cheats or hackmover elsewhere in the game, while doing this quest, don't. At all. That's the easiest way I know to trigger the bug. Walk into the caves the hard way, and be pretty much standing on the staff when you grab it, and you should be fine.

The ghosts at the ruined alchemy shop will spoil the plot bigtime but they aren't actually related to the Xenka bug. (It was thought that the out-of-sequence plot reveal might be messing up the triggers, but investigation of the flags during Exult development showed that it didn't.) Regardless, the ghosts shouldn't actually be giving you that dialogue!

The problem is actually caused by the serpent staff pickup; the "egg" (code trigger object) is particular about how you pick up the staff, and if you do it incorrectly then the flag isn't set and Xenka will never talk about sacrifice because the game thinks you don't have the staff yet.

IIRC, the Exult team planned to fix this by having the code check whether the staff was in inventory rather than relying on the pickup flag, but I don't know whether they got around to it. If you aren't sure about the staff then just leave it be until you've completed every other step to trigger Xenka's dialogue option, then go back there, save the game, pick it up, and go talk to her. If it doesn't work, reload and try again. I know that being too far away is one cause of the flag not being set properly (get close to the staff before grabbing it), and some methods of putting it in your inventory don't work, although I don't remember which they are (dragging it over the in-game sprite, into backpack/bag, other party member, etc).