The ability you're thinking of is called stalwart. It's an inquisitor ability that completely negates the effects of something that has a partial effect on a successful Fortitude or Will save.
You can also get it as an Unbreakable fighter (but only against spells and SLAs) or a paladin with Oath of the Skyseeker, and a Zealot vigilante can select it as a vigilante talent. Mesmerists get astounding avoidance as one of their mesmerist tricks, which is sort of a combination of stalwart and evasion except it only prevents damage.
Animal Companions with the Charger archetype gets "stalwart steed" instead of improved evasion, which is basically stalwart.
