Lovecraftian Master

Ability Scores
We want high Charisma for spellcasting and high wisdom to get our Will save up. Intelligence is not too important, but there is no real reason to dump it. Same with constitution. Strength and Dex are very dumpable, the only problem there being a low reflex score. To that end, here is the ability score layout, with a +2 racial bonus into Charisma.

Str: 8

Dex: 8

Con: 10

Int: 10

Wis: 15

Cha: 20

Put your first ability score increase into Wisdom. Then, you can buff up whatever you like, probably Wisdom or Charisma.

Spells
Level 0 Spells

Level 1 Spells - Level 1 has plenty of small buffs to your Eidolon. Simply putting on Mage Armor and Shield means a +8 to AC.

Mage Armor - At a duration of hours, there is no reason not to pop this +4 AC bonus on.

Rejuvinate Eidolon, Lesser - Curing 1d10 +caster level isn't too impressive. Still, it's useful in between battles.

Shield - Works for minutes instead of hours. Still, if you know a fight is coming it's great to pop on.

Level 2 Spells

Barkskin - Not as great as Mage Armor, but at least it stacks. 10 minutes per level means you won't be tearing through spells to cast it.

Evolution Surge, Lesser - You know what you can do with 2 evolution points? Get two tentacles!

Invisibility - Always Nice. A, very useful utility.

Restore Eidolon, Lesser - As enemy attacks become more complex, this becomes more necessary.

Level 3 Spells

Evolution Surge - You know what four evolution points buys you? Four tentacles! Though if you are already at 9 natural attacks, you may want to splurge for something else.

Restore Eidolon - As enemy attacks become more complex, this becomes more necessary.

Rejuvenate Eidolon - Now it's 3d10 + caster level. Much better.

Stoneskin - Ah, isn't damage resistance nice? Just hope that the enemy doesn't have it!

Magic Fang, Greater - +1 damage and attack for all those tentacles? Yes please! Lasts hours.

Level 4 Spells

Eidolon
We are building some Eldrich being here, straight out of Lovecraft. This means tentacles, frightening, and dangerous. It's not just a concept build. The Dunswitch Horror has plenty going for him, as you will see. And since tentacles only cost 1 evolution point, we are going to have plenty to work with.

We are going to pick the serpentine base form here. We are giving up the ability to pounce, but we are looking for the constrict ability. With plenty of tentacles lashing out, we will have lots of opportunities to constrict.

If you see an evolution that applies to a specific attack, apply it to tentacles.

Evolution surges should be used to make the Dunswitch Horrow grow more tentacles and/or gain the evolution Magic attacks as needed. Just remember you can't have more than 9 natural attacks.

Level (Evolution Pool)

Level 1 (3): Tentacles, Improved Natural Armor, Improved Damage

Level 4 (5): Tentacles x2, Improved Damage, Grab

Level 8 (13):Tentacles x2, Improved Natural Armor x2, Improved Damage, Grab, Constrict, Large

Level 12 (19): Tentacles x3, Improved Natural Armor x2, Energy Attacks, Improved Damage, Grab, Constrict, Frightful Presence, Large

Level 16 (25): Tentacles x4, Improved Natural Armor x1, Energy Attacks, Improved Damage, Grab, Constrict, Frightful Presence, Huge

Level 16 (25) for smaller dungeons: Tentacles x4, Improved Damage, Improved Natural Armor x3, Energy Attacks, Grab, Constrict, Frightful Presence, Spell Resistance, Large

Level 20 (31): Tentacles x5, Energy Attacks, Improved Damage, Improved Natural Armor x2, Grab, Constrict, Frightful Presence, Spell Resistance, Huge

Level 20 (31) for smaller dungeons: Tentacles x5, Improved Damage, Improved Natural Armor x3, Energy Attacks, Grab, Constrict, Frightful Presence, Spell Resistance, Damage resistance 10, Large