There's just one problem; your crits get less and less badass with every level you gain.
|Wow, 2d6 instead of 1d6. Truly I am killed now.|
Well, What Is It?
The book you've been looking for is titled Laying Waste, and it's from 3rd party publisher TPK games. If you want to make your games truly vicious (both for the PCs and for the monsters) then the alternative system this game offers keeps things fast and furious.
|Seriously, look at this goddamn thing!|
I want to take a moment to make it very clear for players who are skimming; this is not part of the core rules, and no one is saying you must use these rules at your table. That said if you want to make your critical hits and fumbles (no crit fumbles aren't part of the core rules either, as I reminded folks in Playing By The Book: Rules Pathfinder Players Often Forget, but they exist in this book) a lot more vicious, this is the book for you.
So How Does It Work?
First of all take everything you know about critical hits and throw it right out the window. Under the Laying Waste rules there is no need to confirm a critical hit; if you hit your weapon simply does maximum damage. In addition to the weapon damage though you roll a d20, add your weapon's critical hit multiplier, and add the amount you exceeded the target's AC by. The result is then compared to a DC 20, 30, or 40 for the severity of the critical effect you deal. You then take the type of weapon you're wielding, and then roll on the appropriate chart. You could do anything from knocking a weapon out of an opponent's hand or temporarily reducing his dexterity to smashing in his teeth or cutting off an entire arm.
|Improved Critical for the win.|
Critical fumbles are more straightforward, but pretty disastrous. When you roll a one you roll on the chart for the potential fumble. You're given a saving throw (such as a Reflex of 20 to get a grip on your weapon again) and a condition (score a critical hit on an enemy to renew your confidence). If you make one or the other then the fumble either never happens, or the effect ends.
What Are The Advantages?
Despite the hate that a lot of 3rd party publishers get, Laying Waste has a definite upside. First and foremost it makes combat more dynamic (I'd hate to use the term realistic when talking about RPGs), and it keeps a genuine threat in every die roll. Even high level characters might be laid low by a lucky shot from a goblin, kobold, or even a commoner.
In addition to honing combat's edge, Laying Waste provides real benefits for martial classes at high levels (since they're the ones most likely to exceed an enemy's armor class by a large number to deal devastating critical hits). It makes critical hits a genuine threat, and a build based around critical hits can have an impact over and above a few extra points of damage on a given swing. It also makes things like concealment and cover more important to combatants, for those who want those kinds of mechanics to get more attention paid to them.
What Are The Disadvantages?
Aside from needing to learn a new mechanic, and then convince your DM (and the rest of the table) to institute this new rule, there are some issues with the system in Laying Waste. While critical hits are a lot more powerful with this system, some players might feel that random chance is taking over from solid character building. The idea of your next natural 20 decapitating the bugbear general is exciting, but if your DM starts dropping a lot of critical hits then a low CR encounter can quickly cut a party into little, bloody pieces.
I wouldn't go so far as to say that Laying Waste completely unbalances a game, because it doesn't. But it can lead to serious problems based on chance rather than on the deliberate actions of players (more so than RPGs already have). It can also really slow down combat any time someone has a critical hit or fumble as pages are checked, additional dice are rolled (and re-rolled for when a given crit simply doesn't make sense or the DM feels is inappropriate).
All in all Laying Waste is a fun little post-market add on for your game. You don't need it to play, but it might be fun to try out. If you notice that it's creating more headaches than heroics though don't be afraid to strip the sprocket out and go back to your old-fashioned critical hits.