Phantasm
These twisted, malevolent spirits are pure evil...literally. They aren't the undead remnants of humans, but undead that are literally made whole cloth from negative energy. At CR 8 and capable of a desecrating aura, possession powers, and negative level drain, the phantasm isn't exactly a creature to mess with. I would personally get more mileage out of them by removing the evil streak, making them merely pure creatures of necromantic energy that aren't evil so much as hazardous to your health if you bother them. That's just my opinion, though.



Phlogiston
Phlogiston, for those of you that are unfamiliar with the concept, is an outdated hypothesis for why things are combustible: namely, the idea that everything has a "phlogiston material" inside of it that produces combustion. So the phlogiston monster is an ur-fire elemental, right?

Wrong! It's a CR 3 Plant that shoots weak fire bullets. Clearly the most logical conclusion to reach, no?



Proscriber
No, it's not your pharmacist's assistant. The proscriber, a CR 15 Outsider, is basically the Cleric's alignment enforcer. While a Paladin risks falling, a Cleric that deviates from their deities' preachings has to worry about a proscriber being sent to punish them, be it through removing class features (including spellcasting) for a divine spellcaster or simply just outright slaughtering. "Oh, you sacrificed a sheep when I stated clearly in my teachings you should sacrifice a goat? Proscribers upon your house!"