If rubidium is installed, killing an enemy with a spirit stealing weapon will cause the game to crash and the world to become unplayable. :(

latest.log

Go to your config folder, look for the client config for ortus lib, and turn off particle buffering.

It worked!! I can't believe it was that simple!

For future googlers who also experience the "Unexpected exception thrown!" issue, hopefully typing that out will lead you here to this solution.

There should be a fix coming soon in 1.4, the issue doesn't happen on the quilt version so we worked out a solution for the forge one too.

Would you suggest changing that setting in general, or only until the fix is available in Malum?

You can just change the setting, it makes the particles render before clouds, which looks bad, but besides that it's harmless.

May I request leaving this issue open? Issue is not fixed - it requires user intervention.
Other issues have already been opened running into the same problem.