Assume Supernatural Ability( Savage Species, p. 30)
You learn to use a supernatural ability of an assumed form.
WIS 13, ability to assume a new form magically,
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. The saving throw DC against this ability is based on your ability scores, not those of a standard creature of the kind whose form you've assumed. For a breath weapon, the saving throw DC is 10 + your Constitution modifier + 1/2 your Hit Dice (rounded down). For all other supernatural abilities, the saving throw (if any) DC is 10 + your Charisma modifier + 1/2 your Hit Dice (rounded down). If a creature, such as an undead, has no Constitution score, use its Charisma modifier or its Constitution modifier (+0 for a nonability), whichever is higher, to determine the save DC. 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.
You can take this feat multiple times. You cannot apply it more than once to the same special ability in order to improve the Will save DC. Each time you take this feat, it applies to a different special ability.
Without this feat, you cannot use the supernatural abilities of a creature whose form you assume.