Evisceration (Ex) A mi-go’s claws are capable of swiftly and painfully performing surgical operations upon helpless creatures or creatures it has grappled. When a mi-go succeeds at a grapple check (in addition to any other effects caused by a successful check), it deals its sneak attack damage to the victim. A creature that takes this damage must succeed at a DC 18 Fortitude save or take an additional 1d4 points of ability damage from the invasive surgery (the type of ability damage dealt is chosen by the mi-go at the time the evisceration occurs). The save DC is Dexterity-based.
This is the ability granting it sneak attack.