Assume Supernatural Ability is a feat that allows a shape-shifted creature to use an ability of its adopted form:
You learn to use a single supernatural ability of another kind of creature while assuming its form through a polymorph self spell or a similar effect.
It's clear how this interacts with abilities that require actions to be used. However, how does it work when I want to assume a passive ability? I've been looking at the nymph's Unearthly Grace (Su) which passively grants bonuses to AC and attack. I'm a doppelganger, so I would assume this is permitted.
Additionally, the feat bestows penalties when the user activates the ability he is emulating:
Using this alien ability is disorienting. You take a -2 penalty on all attack rolls, saving throws, skill checks, and ability checks. Additionally, in a stressful or demanding situation (such as combat), you must succeed on a Will save (DC 19) or be unable to use the ability.
Do I suffer these penalties when attacked, entering combat, or am I always suffering penalties as long as I'm shifted into a nymph?
Best Answer
Assume Supernatural Ability
A general feat (Savage Species p.30) available to be taken by any creature with the requisite Wisdom Score, and an "ability to assume a new form magically," Assume Supernatural Ability is contentious in its interpretation. This is partly because form altering magic or abilities have been fraught with difficulty, and partly because the feat is not descriptive, and gives no examples for clarity.
The Problem with Polymorph
Vague Wording is Vague
Another reason that the feat's use is contested is that the benefit is nebulously worded, stating:
The first sticking point is the '...a single supernatural ability of another kind of creature...' clause. The ways that this can be read are usually:
-or-
-or-
The second sticking point is the '...while assuming its form...' clause. The taking of form is not strongly defined, with players debating how weighted the word 'form' is within the rules. Even the supposedly 'weak' shape altering magic of Eberron's Changeling, Minor Change Shape, is assumed to allow the changeling to take the form of a humanoid, as evidenced by the Racial Emulation feat (Races of Eberron, pp.110-111)
Thus, the answer to the question of whether a Doppelganger can qualify for and use Assume Supernatural Ability is very difficult to definitively answer, but I personally would say 'yes'. Ultimately, the players, including the DM, of course, must decide whether to allow it.
In hopes of balance, I'd probably just add in a clause that only if your HD or Character Level are equal to or greater than that of the creature you're appearing as are you able to assume that creature's form, exactly as with form altering spells in 3.5 rules.
Finally, an Answer!
Regarding the last part of the Benefits section of the feat:
This is a holdover from the 3.0 edition of polymorph other, which polymorph self inherits from. It has a very similar clause, which states (PHB 3.0, pp.236-237):
The feat looks to actually state that you're 'disoriented' (with the same penalties as being 'shaken', by the way...) the whole time you're using the creature's ability. Thus if it's an attack, during the attack action, you take the penalty. If it's a constant (Su) defense or ability, you're always disoriented. Additionally, you must make the save or either lose, or be unable to use, the ability in a clinch.
That's why Improved Assume Supernatural Ability is in the same book. It has a higher Wisdom prerequisite, but states (Savage Species, pp.35-36):
Happy Gaming!