Forge version: 1.16.5-forge-36.1.0
Twilight Forest version: twilightforest-1.16.5-4.0.426
Link to crash log: https://pastebin.com/RYbdabND

I have server where I play with friends with a couple of mods downloaded, i was in the Twilight Forest when I came across a Skeleton Druid, I used the special ability of a weapon from another mod called Mahou Tsukai, the weapon causes a fog to surround an enemy that the can't see out of, i used that on the druid and suddenly the server crashed. Now when ever I join it crashes the entire server for everyone.

Please tell me how to fix this

Looking at the stacktrace, this is all happening in vanilla source code, indicating this issue may not be on our side. Unfortunately, I cannot verify this as the other mentioned mod is closed source.

I would send an issue ticket to their side. Alternatively, the version of that mod is outdated, so I would update first before reporting.

I'm the Mahou dev - I agree its almost certainly not Twilight Forest's fault. I think it's likely a vanilla bug where calling stop() on a goal sets stuff to null that vanilla isn't prepared to have as null.. I'm pushing what I hope is a fix for it, but i couldnt get the crash to actually happen on my own so I'm not sure if it'll work :)