New Summoner Evolution?

[Orncaex]

Alright so lets say The summon in question is something totaly out there in nature, some sort of nightmare manifestation... and the summoner belives it has acid, lava, liquid nitrogen, or some sort of other elementaly charged substance for blood. How would one go about making an evolution for this ability?

It would only need to be triggered upon taking slashing or piercing damage and be melee in range. I have only seen effects like this pop up once or twice with monsters which is a shame.

[Orncaex]

No ideas anyone? Im not looking to make this an official change i am just looking to make this either something to use aginst players or as a player. There are rules for making spells and such so this shouldnt be too far out there.

I am under the impression that One evolution point is more or less worth about the same as a feat with progressive value depending on weither its 2 3 or 4, so with alot of guess work It isn't that far of a stretch to think this one effect should be hard to fabricate.

[Maezer]

As a custom rule I'd go with either:

a)

2 point upgrade to energy immunity of the same type to do 1d6 typed damage to melee attackers (templated like the barbed devils, barbed defense (Su).

b)

3 point upgrade requiring both energy immunity and energy attack of the same type to deal 1d6 typed damage to creatures that start their turn adjacent and make melee attacks against the eidolon (templated like the burning skeletons, burning aura (Ex)) This would have a visible aura that most creatures would recognize as damaging on sight.

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Edit: If I feel the eidolon is particular well played/envisioned/described and not dominating the table, I probably let these go with a 1 point discount for a synergy factor. And I think energy immunity is not particular powerful and has a limiting factor when the best energy defense is the type most commonly resisted as well.