Alchemist | Barbarian | Bard | Champion | Cleric | Druid | Fighter | Gunslinger | Inventor | Investigator | Magus | Monk | Oracle | Psychic | Ranger | Rogue | Sorcerer | Summoner | Swashbuckler | Thaumaturge | Witch | Wizard

Animal Companions | Construct Companions | Eidolons | Familiar Abilities | Specific Familiars | Undead Companions

Become Shadow

Shadow Transmutation 
Source Secrets of Magic pg. 229 1.1
Ability Type Specific Familiar - Shadow Familiar
The shadow familiar transforms its body into barely tangible shadow. It gains resistance to all damage (except force) equal to half your level but can't use any actions requiring a physical form. The familiar can also slip through gaps at least 2 inches wide, or 1 inch wide if it Squeezes. It can use this action again to return to its normal form.

This action has the trait matching your tradition of magic, or occult if you aren't a spellcaster.