as in the augenemted critical Extraordinary Ability possessed by some monters (such as the Tarrasque)?
as far as I know in all cases it triples the threat range (to 18-20) and causes the attack to deal tripple damage rather then double.
well as far as a weapon you could add the Keen property, which doubles the threat range for the cost of a +1 modifier.
It seems to be a kind of special ability for monsters.
The deepwood sniper and the weapon master in the 3.0 edition got a kind of "mega-inproved critical"
In 3.5 I don't see anything, the rules on critical hits are less dangerous now (no stacking).
For example, are there guidelines for making a sword with an augmented critical power (and if so what's the value of it)? How about an advanced PrC ability? Or is an augmented critical just any crit function above and beyond that presented for weapons et al?
Well, I'm not sure how much of an enhancement bonus it would count for, but it would probably be epic (+6 minimum). I suggest you go with a keen mercurial weapon. The Waking Lands had it up, but they seem to have had some pretty major problems with the site recently. IIRC a mercurial weapon had its threat range reduced by 1 (or recieved a -1 to confirm critical hits if the threat range was already 20) and its multiplier increased by 2. So, a keen mercurial greatsword would do 2d6/19-20/x4.
Is that what you're looking for?
EDIT: Oh, and I believe creating a mercurial weapon costs an extra 600 gp, or 1,200 gp for a double weapon. Also, I don't believe you can have a light or piercing mercurial weapon.
The only monster besides a Terrasque that I remember having this ability is Nimblewrights from Monster Manual II.
Anyone remember any others?