Eidolons That Use Weapons

[Rasman]

So, my Eidolon has five sets of arms and wields Falchions.

The question is this, what is his attack progression?

Currently we are 12th level, so his BAB is +9 naturally. I have Weapon Focus and Multiweapon Fighting, so +1 for the Falchion and only a -2 and -6 for main hand/offhand. So, ignoring his Strength Score his Attack Progression should be +8/+4/+4/+4/+4, correct? The DM suggested that he THOUGHT that the progression might go a little differently since a Barbarian has a normal progression of +9/+4.

Secondly, does he also get his Bite as part of that progression? That's his only, real, natural attack that he has outside of the weapons.

[Roadie]

You're looking at the penalties wrong. Take another look at the table on Two-Weapon Fighting (http://www.d20pfsrd.com/feats/combat-feats/two-weapon-fighting-combat---final). The -2/-6 is the adjustment to to the base penalties, not the penalties you use.

In this case, since falchions aren't light weapons, you'd have (not counting Weapon Focus) +5/+0 for the first set of arms and then +5/+5/+5/+5 for the rest.

Then you'd add all natural weapons to the sequence, treating them as secondary natural attacks—which means -5 to attack and you only get 1/2 Str mod on damage.

Note that the rules are ambiguous on how to handle a two-handed weapon that's an off-handed weapon when it comes to damage. The most likely interpretation, though, is that you only get 1/2 Str mod to damage with each falchion past the first.

The full sequence, including Weapon Focus this time, would look something like:

falchion +6/+1 (2d4+Str*1.5) and
... falchion +6 (2d4+Str*0.5) and
... falchion +6 (2d4+Str*0.5) and
... falchion +6 (2d4+Str*0.5) and
... falchion +6 (2d4+Str*0.5) and
... bite +5 (1d4+Str*0.5)
for a total of 11d4+Str*4 if everything hits.

You may want to see about switching to light weapons, since it'd give you a substantial boost in accuracy as well as (given the way the Str bonus damage works) total damage. For example, if I swap out the five falchions for ten kukris (lower base damage but same crit range)...

kukri +8/+3 (1d4+Str) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... kukri +8 (1d4+Str*0.5) and
... bite +5 (1d4+Str*0.5)
for a total of 11d4+Str*6 if everything hits.

See what I mean? Suddenly you've about doubled your chances for individual critical hits (excellent for Critical Focus-derived feats), and you're doing more total damage from Strength, and you're much more likely to hit in general, since you're doing twice the attacks at a higher bonus for all of them.