EDIT:
Uh, disregard I suppose. I Googled "Ultima Underworld lost key item" and found a post here from somebody who dumped the exact same item.
Seems it's not technically required to beat the game, and there's more lying around, so maybe I'll be fine after all.
Sorry! Posted too hastily in my desperation!

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Hi there,
About 20 hours into Ultima Underworld, I've learned that I need Thread from level 1, but I dumped it somewhere along the way due to it not seeming useful, and desperately needing carry weight. No idea where I left it, even with marking my storage areas. Have a feeling I may have even thrown it into water...
I've tried save editors, but they only seem to let you modify character stats, as far as I can tell. Any idea if there's some way to add an item to the inventory?
Thanks in advance.

Yeah, thread can be found in multiple places, and any thread is fine. The place I know off-hand is indeed on level 1: Go North past the Grey Goblin's area, to take the unhidden door on the left near the end of the hall. Cross the bridge in that area and there are a few instances of thread near the back in a corner.

Beyond that, just keep your eyes peeled for more. If you can't find any, then Sanct Flam can get you across lava in a pinch.