Hi!

I don't know if to leave this here or at rftools github, but im going to start here.

Withercrumbs wither can't be spawned with rftools spawner because it doesnt recognize the mob as wither.
Rftools spawner customizes requirements to spawn based on what mob it is and therefore it needs to know what mob to spawn and look it up in its list.

Thanks!
Frosty

I would initially think this would be an RFtools issue, because this mod is just an extension of the vanilla wither, but I would also have to know how the mod handles other modded mobs

The spawner requires the syringe NBT to contain among other things(name and level) mobId="WitherBoss", but with your mod the mob ID is withercrumbs.something_something_wither.
And I'm pretty sure it can only spawn vanilla mobs.

If it can only spawn vanilla mobs then sorry it can't spawn this wither. Like I said, It is an extension off the wither so it is technically a whole other mob.

In Skyfactory 3, all spawned withers are crumbs withers. I am still searching for a way to make a wither farm. All the inclusion of your mod in the skyfactory modpack does, is making my life difficult :D well, and more enjoyable, too...

Wither crumbs are withers. There should be no need to regular withers