Ahnehnois, what this threat is lacking is some solid examples that would make your viewpoint make sense to the rest of us.
Can you build an encounter to show us how it would work? I've got a wizard who is making a deal with a demon. The party hasn't managed to interrupt his ritual in time, so they're arriving just in time to see him complete his nefarious task and become the vessel of a demon within this mortal plane.
How would your system work in making this an interesting and compelling experience for the players?
Um, okay. Guess I'll bite.
"My system" is at the moment a heavily houseruled 3.5, and I did something not terribly dissimilar to this a year or two ago.
Since the warlock (not wizard) was affiliated with an ancient demon lord of vermin attempting to return and there were no obvious choices straight out of the book, I created his servant by applying some spicy templates to a naga, IIRC, and adding some vermin-y descriptions. It was already much more powerful than the party, but I selected spells for it from some of the newer supplements, gave it a couple of nice MIC items, and advanced it several HD (like all my monsters, it also had a custom ability array; since it was built as a "boss", it had stats as good as or slightly better than theirs). It pretty much owned them.
I added in a macguffin element whereby the PCs could still disrupt the ritual and unsummon the thing, split the party with the protagonist being temporarily out of action, one character trapped and about to be sacrificed to the demon, and the third and last character left alone and summoned to leave and return to the druids. Instead, she decided to search for her comrades, discovered the cultists' lair, roused a mob, and sacked the place.
The militia ran from the demon/naga and the party, the main protagonist having been sent back in the nick of time by the fey, defeated the warlock and held off the summoned creature long enough for them to destroy the macguffin and dismiss it. The character on the altar was in pretty dire straits, but it all worked out in the end. The angel on the other side of the room was freed, and it teleported them to their safehouse in the city they were headed towards (which didn't work out so well, but that's another story). I titled the session "Salvation".
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Well, you wanted an example. To the point of this thread, the "boss" naga was not designed by whoever wrote the monster muanal as a boss, and my statblock didn't look anything like what was in the book. It's a miscellaneous monster, but I chose to make it overwhelmingly powerful because I liked the flavor and wanted to scare the PCs. It was not remotely balanced (unlike the warlock, who could also be considered a boss and was higher in level than the PCs). They could never have beaten it in a fair fight. But it was effective for what I wanted it to do.
If they had gotten the sense that it was balanced to provide a particular challenge, it would not have been scary. If they could easily identify it and understand its capabilities after having read its monster manual entry, it would have been boring. Customization-the choice to make it a "boss" by altering its mechanics using various advancement rules-is what worked here.