Something I created that I'd like to share with everyone and open up to thoughts and comments.
Level 20 Half Elf Summoner with all bonouses going to evolution pool. Five Extra Evolution feats. I wanted to make a hydra so I started with the Serpentine base form. Total of 36 evolution points to spend for the Eidolon. Here is the beast I created.
Serpentine
NN Huge Outsider
Init +5; Senses Darkvision; Perception +18
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Defense
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AC 36, touch 13, flat-footed 31 (+5 Dex, -2 size, +23 natural)
hp 157 (+90)
Fort +11, Ref +14, Will +9
Defensive Abilities Evasion
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Offense
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Speed 20 ft., Climbing (20 feet), Flight (20 feet, Poor)
Melee
+1 Adamantine Falcata +20/+15/+10 (3d6+23/19-20/x3) and
+1 Adamantine Falcata +20/+15/+10 (3d6+15/19-20/x3) and
Bite (Bite) +21 (3d6+33/x2) and
Bite (Bite) +21 (3d6+33/x2) and
Bite (Bite) +21 (3d6+33/x2) and
Bite (Bite) +21 (3d6+33/x2) and
Bite (Bite) +21 (3d6+33/x2) and
Gore (Gore) +21 (2d6+11/x2) and
Tail slap (Tail Slap) +21 (2d6+11/x2)
Space 15 ft.; Reach 15 ft.
Special Attacks Energy Attacks (Cold)
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Statistics
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Str 38, Dex 20, Con 22, Int 7, Wis 10, Cha 11
Base Atk +15; CMB +31; CMD 46 (can't be Tripped)
Feats
Combat Reflexes (6 AoO/round)
Double Slice
Exotic Weapon Proficiency (Falcata)
Greater Two-weapon Fighting
Improved Two-weapon Fighting
Multiattack (Multiattack / Extra Attack)
Power Attack -4/+8
Two-weapon Fighting
Two-weapon Rend
Skills Acrobatics +5 (+7 to balance, +1 jump), Climb +22, Fly +15, Intimidate +15, Perception +18, Stealth +15
Languages Common
Combat Gear +1 Adamantine Falcata, +1 Adamantine Falcata;
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Special Abilities
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Climbing (20 feet) You have a Climb speed.
Combat Reflexes (6 AoO/round) Can make extra attacks of opportunity/rd, and even when flat-footed.
Darkvision (60 feet) You can see in the dark (black and white vision only).
Devotion +4 (Ex) +4 Morale bonus on Will Saves vs. Enchantments.
Energy Attacks (Cold) (Ex) Your natural attacks deal 1d6 additional energy damage.
Evasion (Ex) No damage on successful reflex save.
Flight (20 feet, Poor) You can fly!
Improved Evasion (Ex) No damage on successful reflex save; half on failed save.
Multiattack / Extra Attack Multiattack or second attack with primary weapon at a -5 penalty.
Power Attack -4/+8 You can subtract from your attack roll to add to your damage.
Tail (Ex) Tail grants +2 Acrobatics checks for balance.
Two-weapon Rend You deal an additional 1d10+(STR*1.5) if you hit with both of your weapons.
[spoilerEvolution Breakdown]
From base form-
Climb
Tail
Tail Slap
Bite,1
Reach (Bite)
Added-
Head
Head
Head
Head
Bite,2
Bite,3
Bite,4
Bite,5
Improved Bite (1)
Improved Bite (2)
Improved Bite (3)
Improved Bite (4)
Improved Bite (5)
Improved Damage (Bite)
Gore
Energy Attacks (Cold)
Large
Huge
Flight Wings
Limbs (Arms)
[/spoiler]
It's a little weak on defense but is an offensive monster of death - Dual wielding and still getting 1.5x str on it's bites. Could just just a single twohander as well, modify feats as desired. It's fairly easy to scale down to whatever level you wish and still keep the flavor intact.
Comment and enjoy
No fast healing? That seems like an important part of a hydra. Drop the flight and the gore. Also, why do you have improved damage (bite) selected six times? What's the difference between improved bite and improved damage bite?
Improved Bite makes a single bite get 1.5x strength, so I need to take it for each one. Improved Damage (bite) increases the dice step by 1 for all bites.
Ahh, ok. That makes sense. What about the fast healing?
I am not going to lie... This is a crazily awesome idea!
Why a Gore attack?
Gore was added to fill out his natural attacks. Sting might be a better alternative but I don't know if you are allowed to have that + tail slap with only one tail.
Going with only four heads instead of five gives some nice options too. Remove the gore, a head, a bite, improved bite - add wing buffet. Frees up some points for other abilities if you want them.
Fast healing fits the theme, but in general it seems a little weak. 4 evolution points for 1 hp a turn - meh. It's not real regeneration. I guess it's nice in that your eidolon will always be at full health between encounters, if you want to add it then go ahead.
If you drop a head. You could probably get Claws and Wing Buffet.
At level 20 you are limited to 7 natural attacks. As listed they are 5 bites, a tail slap, and a gore. As suggested it would be 4 bites, a tail slap, and two wing buffets. No room for claws, and besides the arms are being used for manufactored weapons so you wouldn't want claws anyway.
I figured that out after it was to late to edit my post...