I'm attempting to create a new speedrun for Ultima Underworld 2. You can follow the progress here:
http://forum.speeddemosarchive.com/post/ultima_underworld_ii_labyrinth_of_worlds.html

Unfortunately I've consistently run into a bug in the game and have no idea what's causing it. Right at the end of the game, after the conversation with Nystul telling him where and when the spell was cast, Mors Gotha is supposed to storm the castle and try to stop you. However, she fails to appear in the castle and I get stuck because Nystul won't continue with the ritual until she has been defeated. This makes the game unwinnable.

The bug seems to be caused by something in the speedrun route, but haven't been able to pinpoint it yet.

Here's a successful run:
https://www.youtube.com/watch?v=MOB3O2uQ_gE

And a run with the bug:
https://www.youtube.com/watch?v=f1i5mvFgkbU

I know there were a few bugs in older versions which prevented MG from showing up altogether, but in the run I do see her once in Killorn Keep and manage to steal the spellbook. It seems that the bug occurring here must be different to those older ones.

Any ideas?

When you steal the spellbook, how are you escaping? Are you fighting through her and her guards or using some other trick?

I found the problem - It turns out that the female guard that likes to hang around in the hallway outside Nystul's room was preventing Mors Gotha's appearance somehow. Pushing the guard down the hallway into the throne room before talking to Nystul allows MG to appear in the castle correctly.

Once this bug is triggered I couldn't find any way to fix it except by loading a previous game.