The material below is Open Game Content from a 3PP Source
Hiss (Ex): An eidolon can make a loud and disconcerting hiss or similar noise as a swift action, forcing air through its teeth, using specialty breathing pores, or using a natural rattle as some snakes have. This sound forces any creature within 20 feet of the eidolon with Hit Dice equal to or lower than the eidolon’s to make a Will saving throw (DC 10 + one-half the eidolon’s HD + the eidolon’s Charisma modifier) or be shaken until outside the hiss’s area of effect. This is a sonic attack. A creature that is already shaken, frightened, or panicked is immune to the effects of a hiss.