Hmm... I think I like where this PrC is going. Like you say, it's kind of like the Ventrid Swordsman, with a dark twist and scythes instead of swords. Pretty cool!
I'll address the flavor first: You make it sound like Grim Reapers just riddle the streets of Enworld. But to be one, you have to be reasonably powerful, and to be a powerful one, you have to be above L12. We've established that there are few such powerful characters in Enworld. I'm sure there are some fully progressed Grim Reapers in the world (and actually, I'll address this more later), but they are few and far between, and even the lower level ones should be rare. So basically, I think you need to reword it a little bit to say that Grim Reapers are less populous.
Another thing that's just personal, take it or leave it: I think they should be more affiliated with Seto. I'm sure some Grim Reapers are arrogant and full of pride and consider themselves the source of their power, but I also think that pretty much every GR probably pays at least some small homage to the God of Scythes. I certainly don't think too many, if any, GR's completely shun the god.
Now for some mechanics!
First and foremost, I think Greater Demoralize should simply be a required feat for the PrC, rather than having the contingency paragraphs in all of the improved demoralize abilities. A Grim Reaper would be foolish not to take GD. It shouldn't replace Persuasive, because it doesn't serve as a sacrifice feat. ... Though, Persuasive is not so bad, with all these new ways to use Intimidate. Which brings me to my next issue:
I think this should be a 5 level PrC. I'm not crazy about Skill of Fear, thematically or mechanically, and I feel like Frightful Presence just takes Intimidate to a whole new level, beyond what a simple skill should be able to do.
Unnerving Presence doesn't make sense at all, because Handle Animal is one of the Grim Reaper's skills!
I think Reaper's Blade is much better than Reaper's touch. Keen edge on scythes is good, that x4 crit suddenly becomes twice as likely to happen. So I think you should switch those two abilities, drop Reaper's Blade to L3 and Reaper's Touch to L5, and knock off the last 5 levels. Something like this:
Code:
GRIM REAPER (d10 HD)
lvl BAB Fort Ref Will Special
1st: +1 +2 +0 +0 Expert Demoralizer
2nd: +2 +3 +0 +0 Bonded Scythe
3rd: +3 +3 +1 +1 Reaper's Blade (+1 dmg)
4th: +4 +4 +1 +1 Master Demoralizer
5th: +5 +4 +1 +1 Reaper's Touch (increased crit)
Also, I don't think that Bonded Scythe needs that craft DC. A Grim Reaper would just take 10, unless he had a low Int. I think it's a needless penalty to make stupid Grim Reapers waste more than 1 day bonding with their weapons. Such characters are already spending 5 skill points on something they'll probably rarely use.