Eberron Monk 11/Disciple of Dispater 8/ X 1
With the feats Whirling Steel Strike (Eberron Monk), Lightning Maces, Improved Critical, and Roundabout Kick. Maybe that feat that gives you an extra attack with that one exotic weapon from sandstorm. Also, you can grab some incarnum abilities to give you Sphinx Claws, and thus pounce, but it isn't required.
If you run out of feats, have a friendly wizard Embrace/Shun some of the bonus feats you get away.
Then, make this your weapon: an Aptitude Illusion Bane Impaling Mage Bane Maiming Rapier.
Grab an item that gives you haste at will. Shouldn't be too expensive.
End result:
1. You can flurry with your rapier. Your rapier crits on a 9 or better, ignores miss chance from illusions, can act as if it's making a touch attack 3/day and it really hurts casters.
2. Each time you threaten a crit, you get an extra attack. Each time you actually crit, you get another attack. It's more likely that you get an extra attack than not.
3. Even if, by some freak circumstance you don't threaten a critical, at level 20, should X be a full BAB class, you have 8 attacks on a full attack, at the following numbers: 15/15/15/15/15/10/5/0.
Doing some quick calculations, the chance that you don't threaten a critical at all is 256/390625, which is rather low. In fact, you should be threatening about 4 criticals or more each full round action, which would get you 4 more attacks, each with the sam chance of threatening a critical...
It's just a little something.