There are two abilities with this name, neither of which have the DC listed in the official description. The monster ability has a DC of 19, while the shifted ability (from the mindflayer form of greater wildshape IV) has a DC of 10 + (shifter level)/2 + wisdommodifier.
If a target is already stunned by this ability, it will be damaged instead. The damage is 1d6 magical damage for every 3 shifter levels if the caster is a shifter, and 1d6 magical damage for every 2 hit dice otherwise.
The shifter version of this ability has a scripted limit to its number of uses (1 + 1/3 shifter level for each shift).
Saving throw bonuses versus spells (besides the bonus from spellcraft) do not apply against the shifter version.