I went down the central staircase in the temple, but the stairs don't exist on the lower floor so I can't go up. Is this a bug or some kind of a trap? And if it's the latter, why don't the characters say anything about it?

Also on this floor is a mirror where an angel of Iuz or whatever says he can enchant my weapons. This seems shady for sure, but I don't understand how to even do it, since there is no container in the room and just leaving them on the floor does nothing...

1. The stair is a one-way door. You are in a lot of danger right now. The challenge is now to find your way back up to the exit and safety without getting a TPK. It's time to put those thief sneaking skills to the test. This happened to me the first time I played ToEE, and beleive me, if you pull it off you are in for the most memorable playthrough experience you will ever have. There are several other ways to get into the lower levels of the temple without knowing the way back out as well.

2. (SPOILER ALERT) Just walk around the dungeon some more without any weapons if you want to see what happens...

Good to know it isn't a bug at least. It's strange the characters don't say anything when a staircase disappears. I did notice there were some hard monsters here, but I did manage to kill trolls and things like that. I don't save if someone gets killed so think I'll try to find a way out rather than loading an old save.

I actually figured a relatively easy way for getting out of here, although it probably was possible only because I'm evil.

What does 'ability loss' mean? The hag hit one of my characters with this, and now she's encumbered by her own equipment. I thought it meant stats getting drained, but when I loaded an old save and looked, and her stats are the same as they were before.

EDIT: OK, that was just a glitch. I took her armor off and put it back on, and the encumbrance disappeared...

^^

Yeah there's an issue with that. You might have taken ability loss earlier, but the game doesn't always update the penalty when your ability recovers. Switching gear does seem to fix it, or moving/dropping a backpack item.