All of the above. The intent is when they reach a certain corruption level (tempted into it) they gain an extra feat. It is also intended that the feat feels associated with the type of corruption. I.e. void would be different than rage.
Player and DM work out flavor.
They gain feats as they gain corruption, its a bribe/temptation for MOAR POWAR. Through gameplay. (edit)
"Do I use the forbidden spells, and risk getting closer to oblivion and our fuzzy lord Hastur?"
I've been using 1-2, 3-7, and 9-10 as the tiers.
But using proficiency bonus is interesting, it would allow for weaker characters to be overwhelmed quicker (good), and "stronger" characters to endure.
Hmmmm.
So if a PC had a 12 Con, they could get 6 free feats and start showing changes, with a total of 12 before becoming and NPC?
That much power would be tempting indeed. Seems a bit high. My players would gladly take 5 free feats and say "thanks", never gaining another one.
Valid, I reckon I was thinking about it from the view of someone who has seen those kinds of things over time and gotten used to it.
For new viewers? Yeah, you are correct...its well put together and scarey.
It was scarey when I first read it when I was 10 or 12.
Much like Jaws was terrifying and hundreds of people were scared of the ocean after seeing it...and now its very tame and we laugh and meme it.
Funny story...
Re: Overthulhu Saturation
When I created the basic outline of my campaign world in the mid 80s (and its secret history etc) I had read Lovecraft and Moorcock extensively.
So I created the campaign, and the gods were more like immortals from basic dnd, and not directly...