Re: the Arachnidal Rebirth, I'm just not over the lack of restriction in targets. Can you use it on a ghost? Does the ghost's head stay incorporeal? Clockwork constructs with their mechanical heads now attached to a living spider body?? Does a beholder get an entire little beholder to use as a head now??
As written? There doesn’t seem to be any restriction aside from the fact that your target has to be able to die. A ghost, many forms of undead, and constructs technically don’t “die” so much as just cease.
I think if you did it on an undead being–including an incorporeal one–they would come back as whoever they were before they died, except Now A Spider. I doubt your DM would allow Forced Reincarnation (and Arachnidal Rebirth) to work on non-corporeal or non-living targets, though, because then it just gets weird. What does a machine reincarnate into?
I’m at a loss of how to handle Beholders and creatures like Cacodaemons, which are technically just floating heads. That’s………… yeah. that’s weird.