I can see possibly using Ambient Fear in stages as the PCs advance in level, starting with Ambient Fear levels 1 - 2 for 1st-level PCs, adding AF level 3 for 5th level PCs, AF 4 at PC level 9 ... and then there's a problem (maybe just a typo):
The description states that there are 5 levels of Ambient Fear with level 4(?) being the most fearsome, with no explanation for AF 5. What's the 5th level then; the moment when your PC starts telling itself, "It's only a game. It's only a game. It's only a game ..."?
Assuming the text should read that level 5 AF is the most fearsome, I'd bring that into the campaign when the PCs reach level 13. I like the idea of scaling up the ambience over time.
As to Terror ... wow. [That's lower case and without an exclamation point because it took my breath away.] I like to think of myself as a relatively kind-hearted DM, never having gone past killing more than half of an adventuring party in any one adventure.
Dead of a heart attack after failing the saving throw against Terror by 5 or more and then failing the following Constitution saving throw? In a game where weapon damage in any given combat can kill a PC as a normal part of play, this shouldn't have such a visceral affect on me (it's still just a game, after all) but it does.
If I should choose to use this option in a campaign (and, oh-my-oh-my, does my lizard-backbrain want to) I would start it all by having the players create a stable of PCs as a pre-campaign bit of foreshadowing that they need to buckle their seat belts: it's going to be a Bette Davis kind of ride.
Thanks, TreChriron.
R.