While loading a world with Eldritch Mobs installed it crashes, this does not happen with the mod removed, seems to be the out of date Polymer library or something; adding the up to date Polymer version causes the issue to poof.

[05:14:30] [Thread-38/INFO]: [EMI] Loading plugin from polymer
[05:14:30] [Thread-38/ERROR]: [EMI] Exception loading plugin provided by polymer
[05:14:30] [Thread-38/INFO]: [STDERR]: java.lang.NoClassDefFoundError: dev/emi/emi/EmiStackList

Which version of polymer did you have to add in order to fix the issue?

I just added the latest at the time, 0.2.28+1.19.2

Fixed in latest release

Nice c: