Hello!
The item "Earth God Essence" which is a Relic for the Earth domain, appears to have a bug in how its polymorph functions. The item description suggests it'll turn you into an elemental, and the spell name even says "Custom", but when used it turns you into a Giant Spider. I tried everything I could think of, thinking the option for the elemental might in sub-menu, kind of like protection from alignment, but it doesn't seem to be.
This is the item in question:
(https://i.imgur.com/YXhVvJY.jpg)
User error, or bug?
Follow up!
Trying the new "/c polymorph ______" commands works, allowing me to use any form normally available under Polymorph Self. If the custom form suggested by the item in question is available, none of the various guesses I made as to its name worked, and it's not listed under the help.
Shoot me a PM stating where you found it, and I'll have it fixed and replaced. I am not sure how that variation of the spell with 'custom' functions but it was not part of the modifications I made.
Should be fixed. If anyone has trouble with this item contact a DM.