An eldritch sendoff to a high-fantasy (or another genre where gods would exist) party is such a cool idea. Because it's system-agnostic, it requires some work from the GM to create statblocks, and adapt the adventure to match the universe's canon and tone. But what a way to end things. It's worth it in my book.
Now to find players who'd be willing to watch this soul-crushing experience unfold. Anyone wants to witness their beloved character slowly fade away into oblivion? There's a cool job application waiting for you if you survive. I mean if you manage to cling to existence.