The deadly bosses don't check for null attributes when they spawn. For certain mobs that may miss certain values, e.g. sheep that don't have attack damage, or AoA mobs that don't have armor values (I think), the following crash occurs:
https://pastebin.com/dvUj6DcZ

Mobs that are not instance MobEntity (like sheep) are not supported for the boss system.
Looks like the error detection could use some work though.

The issue was generated with AoA. I'm not sure why it happens, actually.

If you could source what entity causes the issue, I can try to figure out why it causes it. Chances are it's not an EntityMob, because those attributes are part of that base class

Hmm. I believe my entire config was EntityMob... but I see now, ranged mobs aren't assigned an attack damage in AoA. Side note, is there a way to disable the enchantment tweaks, but keep the enchantability tweaks? Specifically, I want the enchantment table to cap at level 30, but it only gives me low level enchants if I do so. Also, Quark's ancient tomes cease working with enchanted books, I'm not sure why.

You could achieve it by doing a bit of config work, there's no simple "enable half of the ench module" system, so you'd need to adjust all the levels and max powers back down to original.
As for the quark tomes, i'm not sure why they're broken, I'd have to look into quark's anvil update handler.

Closed via Shadows-of-Fire/Placebo@4cbbd56