Warrior of a Thousand Attacks of Opportunity (3.5e Optimized Character Build)
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I had this idea because I wanted to counter-build against a dwarf barbarian that killed my bard. It was kind of my fault... but still. Anyway, the sheer volume of attacks put out (up to 8 in one turn if you full attack) will kill about anything that you can have thrown at you. And since you are loyal beyond death at level 20, nothing can kill you.
PHB II, PHB, Complete Adventurer, Complete Warrior, Races of Stone (I think I got all of them)
 Game Rule Components
None really necessary, besides armor, shield, and weapon, but anything that will increase your Strength, Dex, and Charisma is welcome.
Starting Ability Scores (Before Racial Adjustments): Str13 Dex15 Con16 Int 8 Wis8 Cha 16 Race (Templates): I was human, but about any race will work, something with natural armor would be nice, since knights don't get tower shields.
Starting Racial Traits:
|Saving Throws||Feats|| Class
|1st||Knight||+1||+0||+0||+2||Shield Specialization, Combat Reflexes||Fighting Challenge +1||<- other stuff ->|
|2nd||Knight||+2||+||+||+||Mounted Combat||Shield Block +1||<- other stuff ->|
|3rd||Knight||+3||+||+||+||Hold the Line||Bulwark of Defense||<- other stuff ->|
|4th||Knight||+4||+||+||+||<- feat(s) ->||Medium Armor Mastery, Test of Mettle||<- other stuff ->|
|5th||Knight||+5||+||+||+||Bonus: Ride By Attack||Vigilant Defender||<- other stuff ->|
|6th||Knight||+6/+1||+||+||+||Expert Tactician OR Deft Opportunist||Shield Ally||<- other stuff ->|
|7th||Knight||+7/+2||+||+||+||Fighting Challenge +2||<- other stuff ->|
|8th||Knight||+8/+3||+||+||+||Call to battle||<- other stuff ->|
|9th||Knight||+9/+4||+||+||+||Active Shield Defense||Heavy Armor Mastery||<- other stuff ->|
|10th||Knight||+10/+5||+||+||+||Bonus: Spirited Charge||<- class features gained ->||<- other stuff ->|
|11th||Knight||+11/+6/+1||+||+||+||Shield Block +2||<- other stuff ->|
|12th||Knight||+12/+7/+2||+||+||+||Robilar's Gambit||Daunting Challenge||<- other stuff ->|
|13th||Knight||+13/+8/+3||+||+||+||Fighting Challenge +3||<- other stuff ->|
|14th||Knight||+14/+9/+4||+||+||+||Shield Ally Plus||<- other stuff ->|
|15th||Knight||+15/+10/+5||+||+||+||Defensive Sweep, Bonus: Trample||<- class features gained ->||<- other stuff ->|
|16th||Knight||+16/+11/+6/+1||+||+||+||Bond of Loyalty||<- other stuff ->|
|17th||Knight||+17/+12/+7/+2||+||+||+||Impetuous Endurance||<- other stuff ->|
|18th||Knight||+18/+13/+8/+3||+||+||+||<- class features gained ->||<- other stuff ->|
|19th||Knight||+19/+14/+9/+4||+||+||+||Fighting Challenge +4||<- other stuff ->|
|20th||Knight||+20/+15/+10/+5||+||+||+||Loyal Beyond Death, Shield Block +3||<- other stuff ->|
 Other Components
Anything you want to add on that increases your main stats, namely Charisma, Dexterity (key), and Strength. I like to pack a longsword because it's good at criticaling, but you can pack any weapon you want.
This build lets you do loads of attacks each round at your full attack bonus. The main reason this works is because of Defensive sweep coupled with Bulwark of defense, and Robilar's gambit coupled with Test of Mettle. What you do is force the enemy to charge you while you hide other party members behind you. Hold the line provides an attack of opportunity against the enemy. Then you issue a test of mettle, and fight using Robilar's gambit. Each of their attacks will provoke an attack of opportunity. If they try to leave, they can't because bulwark of defense prevents them from taking a 5 foot step (and consequentially a full disengage is also not possible. Defensive sweep gives you an attack of opportunity if they don't try to move.
You can also make attacks of opportunity while fighting defensively or in total defense. Active shield defense lets you do this, and I can only assume that it stacks with Robilar's gambit, and that effectively puts your AC back to normal from the +4 bonus your opponents get when you use the gambit. You can also use your Fighting Challenge and Shield block, though those challenges should generally be reserved for tough enemies, but the shield block is free. This adds up to 1 attack of opportunity per round against each enemy MINIMUM, up to 4 enemies, without any modification to your Dexterity. If they attack you, then you get even more attacks. And then you get to full attack them.
Now, wearing Breastplate and with a +3 Dex bonus, holding a heavy shield, your AC is 21. If you equip heavy armor, your AC will become 22. If you can manage a higher Dex bonus, you can even wear light armor and maintain an AC greater than 20. This uses completely mundane equipment. This, coupled with a D12 hit die and a +3 con bonus will keep you alive through most fights. However, under the rare circumstance that you are in over your head, you just get healed by your cleric, bard, or druid. Large enemy bugging you? have the Rogue backstab it. Large mass of enemies angering your? Issue a Test of mettle and blow them away with the help of your friendly neighborhood wizard. This is basically the ultimate protector/support dude. And they can even stand right in the way of danger, and your shield ally ability can prevent them from getting hurt too much.
 Munchkin-Size Me
At 15th level this is at high efficiency. Your 18th level feat and attack bonuses gained will help you make more attacks, but...
 Side Notes
Don't forget your Daunting Challenge: You can make your enemies shaken unless they pass a will save as a swift action, within a 100 foot radius centered on you.
Since your knight's challenge abilities are language dependent, they are ineffective against unintelligent beings. works best on a battlefield. You can still do this against animals, but you don't receive several bonuses that are key to this working, like your Fighting Challenge. You also can't force animals to attack you, but if the party stands behind you like they should, this still works. No matter what, you still put out several attacks per turn at your full attack bonus and cripple enemies you hit, if you take expert tactician.
 DM Counters
Toss more unintelligent things at you, maybe invisible creatures as well. It's not really abusing anything, just clever. Also an opponent with elusive dance (found in dragon magazine 315) can utterly shut down what this build is designed to do.
<- Any other side notes that don't fit in the above sections. ->