Level Up (A5E) Embody Fiendish Spirit question on save DC

[lichmaster]

i mean, i guess you could argue that as you grow in power you attract more and more powerful spirits, but then i'd say that should be tied to upcasting and should come with more benefits (surely a more powerful spirit would be able to do more for you, no?).

Yes, plus the DC should still come from the creature's power. Of course if you can upcast its power is still related to your own power, but using your own DC would be weird IMO.
Imagine summoning a demon, and creatures have to save against its powers but using your own spellcasting DC because that's how it was summoned in the first place. It mixes up things too much IMO