The game crashed whilst initializing game
Error: java.lang.NullPointerException: Registry Object not present: forbidden_arcanus:soul

crash-2020-04-11_22.16.15-client.txt

Could you please try if this also happens with only forbidden & arcanus installed?

I had the same problem, but it was only after I installed scaling health to my mod pack. However, scaling health is not the cause paired only with this mod. I have been adding mods to a test mod pack one at a time to see what was causing it but I gave up after adding all the mods that I think would have even remotely interacted with Forbidden Arcanus.
crash-2020-05-05_11.59.51-client.txt

It crashes with the exact same message on the latest Forge version 31.2.0

MoreCrashInfo mod logs:
crash-2020-05-17_21.09.12-client.txt

traverse: Legacy continued, as of version 3.1.0, crashes with forbidden arcanus making this error.
crash-2020-05-17_16.33.40-client.txt

traverse: Legacy continued 3.0.0 worked fine with this mod.

got the same problem.
crash-2020-05-01_11.22.46-client.txt

Should be fixed in 1.15.2-1.4-beta-1