Just as an aside, to show one of hte places where Knockdown can SHINE (this is a pretty cheesy combo, bear with me):
Knockdown and all prerequisites, Spiked chain (including specialisation in it), and Combat Reflexes, and a Strength score of 18 or more. Dexterity of 13+ optional but very useful; Intelligence 13+ required for some feats.
Specifically, presuming a straight-class human fighter:
FEATS
... Racial; Exotic weapon, Spiked Chain
... Ch-1; Expertise
... Ftr-1; Weapon Focus, Spiked Chain
... Ftr-2; Improved Trip
... Ch-3; Knockdown
... Ftr-4; Weapon Specialisation
... Ch-6; Combat Reflexes
... Ftr-6; Expert Tactician
ATTRIBUTES for this example; remember, at Level 4 a single +1 is gained:
... STR; 18
... DEX; 14+
... INT; 13+
RESULT:
A Fighter-6 has a BAB of +6/+1
18 Strength is a Strength modifier of +4
... that's +4 attack
... with a two-handed weapon, that's +6 damage
The Spiked Chain is a 2H weapon doing 2d4 base damage
... with Strength and Specialisation, that's 2d4+8
... ... which is a range of 10-18 per blow
... ... whichis an average of 15 per blow
... with a +1 weapon, it becomes 2d4+9
... ... which is a range of 11-19 per blow
... ... which is an average of 16 per blow
The fighter's net to-hit, assuming a +1 weapon, is +12/+7
Combat reflexes and Dexterity 14+ produces a total of 3 Attacks of Opportunity (AO) per round, maximum.
Expert Tactician produces (errata) an extra melee action during any round you deny an opponent their Dexterity bonus to AC
...
So. The Spiked Chain is the only PHB reach weapon that can also hit foes only 5' fom the wielder, wihtout the wielder having to move at all. This is key; this combination works VERY well when defending a narrow-ish (up to 25' wide) corridor, bridge, or what have you; it is an excellent combination for holding ground (and not too shabby at
taking ground either).
The trick is this: when the round begins, Combat Reflexes allows you to take AO's even when flatfooted; thus, anything that closes for melee with you during a surprise round, if the chain is ready, suffers an AO; at 8th level, take Quickdraw, and even that requirement might be suspended, at the GM's discretion.
If you hit (you do NOT want to use Power Attack here, you want to whore for attack bonusses like there's no tomorrow), you will cause at LEAST 10 points of damage; this allows the Knockdown feat's free trip; if you trip, Improved Trip allows a free follow-up attack. Also if you trip, prone creatures are denied DEX bonusses to AC, which gives you the partial action from Expert Tactician, by which you can whack them again. After potentially gettign whacked three times with a 10-18 damage attack (sans weapon-based enhancements; many 6thlevel fighters will have a +1 to +3 magic weapon) as part of a single AO, the target of your affections should be feeling quite unhappy, if it is still even
mobile. Subsequent AO's willnot produce a partial action, of course, as Expert Tactician will have already been used for that round.
Special note regarding Knockdown and Improved Trip: IMO, and IMC, a prone creature cannot be tripped; the rules may or may not support this, but I am NOT trying to create an infinite-loop combination!
Special note regarding tripping an opponent, and Expert Tactician: while merely being Prone does not deny one their DEX bonus to AC (PHB page 132, table 8-8), being "off balance" does (same page, same table, one entry down), and IMO and IMC, being tripped does definitely count as being put "off-balance" until that character's next action; the victim of the trip has, after all, just been ignominiously dumped upon their petard, against their wishes ...
Now, here's where the tactics comes in. On your action, if you don't NEED to advance ... READY ACTION. Specifically, ready to "whack the first guy to cross X line *draw or indicate line on battle map* and still be standing". The idea is, now you have (due to the readied action) FOUR tries to stop people from crossign a line 5 feet ahead of you, and twenty five feet from side to side -- hurting them grievously in the process. If one of the first three tries fails, that *will* trigger the Readied Action, sine they will "still be standing".
Now, all said and done, you've a pile of folks on their backsides at your feet. It takes a move-equivalent action to stand up,so they won't be using Full Attacks against you, unless they remain prone. If they do, you get a +4 to hit themyourself, and they suffer a -4 on their attempts to hit YOU (again, table 8-8 on PHB page 132). Also remember, by the way, that you will get that +4 to hit for that single extra melee action produced by Expert Tactician, if you use it on someone you've just knocked down.
Another benefit is, on their first round -- they have laready moved, so, to stand up will result in a double-move, and not allow them to make an attack at all. Otherwise, they remain prone, suffer a -4 penalty to attack, and give you a +4 to hit them during the next round.
Magic weaponry makes this even more fun; a key arrangement IMO would be +1, Sure Striking, and Ghost Touch. Bonus damage is nice but not neccessary; as long as your minimum damage is 10 (Sure Striking helps insure that), you're all set. With a +2 weapon enhancement, or, a 20 strength and a +1 weapon, you don't even have to specialise -- freeing up one or two feats (Specialisation and Focus).
A good suggesation for a choice of magic Spiked Chain (a
Sure-Striking Spiked Chain +1 of Ghost Touch -- a real mouthful, eh? be smart, give it a fancy name, if only for brevity's sake
) is thus a market-price +3 weapon, or, 18,000gp, plus cost of MW chain. Get a bit fancy, using
Magic of Faerun, and make it of an extraordinary material ... I would actually suggest "Living Metal" for it's self-regenerative properties, aiding you against hte nigh-inevitable Sunder attacks a DM would throw against a monster so dependant on his weapon. The total cost is then (including Livign Metal construction, shich satisfies Masterwork status for enchantment) 19,525gp; pricy, so not available until at LEAST 8th level (possibly later, since even at 8th level it's most of the characetrs wealth-allowed), but it's all the character will ever NEED to have, period (barring ELH enhancements,which I have not yet seen, and so, cannot speak to). Dropping the Ghost Touch lowers the price to 9,525gp, affordable for 6th level, fairly reasonable for 8th level, and makes sure that only incorporeal creatures will be resistant to your attacks (how
do you trip a ghost, anyway?).
I'd also suggest some nonmagical backup chains. Well-hidden-upon-your-person ones.
Such a character can, for style, choose to gain the Master of Chains Prestige Class somwhere after 8th level, with relative ease. However, right from 6th level, when THIS black knight says "NONE SHALL PASS" ... he means it!!
...
Now ... does
THAT increase folks' appreciation for the Knockdown feat?
Note: aside from the issue of reach, most of the benefits of putting people prone can be had with other high-damage-output weapons, especially 2H weapons like Falchions and Greatswords.
Also note, normal-reach weapons can produce the AO as they LEAVE your threatened area to close to 5', but you need to step 5' back; for characters wishing to use such weapons, I would replace the "Exotic Weapon" feat with "Hold The Line", which requires rearrangement of the entire feat selection list but it can still be done.
