I don't know whether it's open source, but there's a guy doing a Unity port: https://github.com/hankmorgan/UnderworldExporter
Are there any Open Source ports for UU in the vein of zDoom and OpenXCOM? I am looking online but so far it's not baring much fruit, and was wondering if perhaps I'm not searching for the right terms, even though "ultima underworld open source" seems like a good search term.
I don't know whether it's open source, but there's a guy doing a Unity port: https://github.com/hankmorgan/UnderworldExporter
There have been many attempts over the years, but it's an incredibly difficult thing to attempt without the original source code, so the projects inevitably stalled at one point or another. I think the UnderworldExporter project is the only one I've ever seen that looked likely to result in a complete working game, so I'd say that's the one to pin your hopes on :)
There's version 1.0 out that should be playable from start to finish and has all the core systems implemented, so I'd say the chances are pretty good for this one.
Also, looks like the source code for this is available as well: https://github.com/hankmorgan/Underw...eases/tag/v1.0