Well, to start, the requirements for this don't make sense, mechanically or thematically. Knowledge (Weaponry) is not a valid basic knowledge, nor can I find it's introduction elsewhere (it's not in Complete Warrior nor Tome of Battle).
And the feat requirement is quite silly. On one extreme it's a wizard that has to take one feat for simple weapons, one feat for
one martial weapon, and a third feat for
one exotic, who then suddenly is proficient in
all weapons, including exotic, and on the other extreme fighters still have to take the feat because they are never explicitly granted it (or welcome to Fighters DCFSing away all but one of their martial weapon proficiency feats, and then doing the same repeatedly after entering the class).
And if you really want to use it, you need to move the fluff text under "becoming a living arsenal" to a special segment under the requirements or to a class feature. Requirement would limit entry, feature would (most likely) deny this class's features (I'd recommend for a day) if using the prohibited actions; however, that list of prohibited actions is quite silly, as it would cut off all access to even use activated items and all the initiator classes that would actually work well with this class concept.
Need to change the skill requirement to something existing, I'd recommend Craft (Weaponmaking).
And remove the feat requirement for a Special requirement of the same meaning, like: "Proficient in all simple weapons, at least one martial weapon, and at least one exotic weapon."
The skill list is far too small, as anyone with any Int bonus will literally not be able to take as many skill as they gain skill points. And see above for the Knowledge.
And the class gains no hitdice. I'd recommend d10.
Now, for the features:
A Living Arsenal has proficiencies in every single weapon it is possible for any humanoid species to wield.
Why? Their anatomy already prevents them from using anything else, regardless of proficiency. Replace this with "A Living Arsenal is proficient with all weapons." And then realize that it's a silly thing to do when you've made them waste feats on this exact thing to even enter the class.
Weapon Understanding (Ex)- A Living Arsenal understands the only real difference between a staff and a sword is that one of them is sharper than the other. Any time he takes a feat that normally only applies to one weapon, it applies to all weapons that it is applicable to.
Wording on this couldn't be worse. When the feat only applies to one weapon, it is only applicable to that one weapon.
I'd recommend replacing both of these with Weapon Aptitude and Exemplar Aptitude from
my Fighter. Or just use them on your Fighter and make half this PrC obsolete two levels early, which I think is a good note of it's power (or rather, lack there of).
Hell, even replace it with the Warblade's Weapon Aptitude (which is still written better than Weapon Understanding, despite it's own vagueness) and forget the Exemplar, it'd still be an improvement.
Bonus Enchantment (Su)- A Living Arsenal begins to subconsciously tap into their own latent magical power...
... and loses all his class features because he can't concentrate enough on weapons. Brilliant.
No retraining method makes this suck mightily, and it doesn't say which enchantments can be selected nor when he can select them. Rewrite this so it makes sense. As it is, the intent is so unclear I don't even know if it's too weak or too powerful, but both are entirely possible. Is the total adjustment capped at 1/2 his level? Each individual enchantment? Does this allow him to go over +10 total pre-epic? Does this stack onto already magic items? Can he pick normal + boosts?
Undeniable Grace (Ex)- A Living Arsenal is extremely graceful with any weapon of his size or smaller.
So graceful with a greatsword but not a Large sized dagger, which is still smaller and lighter? Just eliminate that, larger weapons are a trap anyway.
When making an attack, the Living Arsenal adds their Charisma bonus to their attack and damage rolls.
Cha to damage AND hit is nice, especially for only 3 levels, but doesn't actually make up for having to take 3 levels in this class.
Also, on a critical hit, all enemies that can see the Living Arsenal get a -2 moral penalty for the round.
'
To what? Attacks? Damage? Saves? AC? Checks? Hit points? Basket Weaving?
Also needs editing for clarity and readability. Decent idea, but also needlessly complicated. Could be replaced by a simple: "Can hit adjacent foes with reach weapons and can throw any weapon with a 15' range increment. Additionally, all weapons used deal piercing, slashing, and bludgeoning damage." And no, that really is not overpowered, and it stops the silliness of "my dagger is a reach weapon? That can't hit adjacent targets???"
When attacked physically, the Living Arsenal can do one of the following: he can perform any of the special attack actions he is capable of using normally,
He already can, since he is normally capable.
Why? This is a straight nerf. Even if it's meant for all attacks to provoke an attack of opportunity (a much better way to do what I think you were trying to do), why do his options from that need to be nerfed?
Replace this with granting Robilar's Gambit or Karmic Strike as a bonus feat and granting an unlimited number of AoOs.
or he can add his BAB and Strength/Charisma modifiers to his AC.
So, he can add BAB, Str,
and Cha to AC? That's a little absurd and
wayyyy more powerful than the other part of this class feature. That is, by itself, 3 quite powerful features, especially if they are unlimited by Max Dex of the armor he is wearing. Even for a poorly built melee character, this feature will add at least +20 to AC. For a decently made one, you're looking at +31 or more, and if you're actually optimizing it that's going to be topping +40.
And deny him his entire class of abilities, including this one. Entirely contradictory and not very powerful. Plus it's a /day limit, which are silly anyway. Replace this with just making any weapon he wields a +5 weapon and allow him to lose the affect for X rounds to give it to up to 20 allies for the same X rounds. It would still be a crappy capstone though.
This class, as (poorly) written now, is quite a trap with 2 amazing class features that don't actually make up for the badness of the rest. Those features are great material for an IS to eat though.
Is English not your main language? You (or someone else for you) really should rewrite this for clarity and more standard wording.
And then you should rewrite it to not suck. As is, going straight Warblade or straight through one of the many Fighter fixes (mine linked above included) are much better than ever taking any levels in this class.
Now, tear this baby a new one!
Done and done.