You can spend 10 minutes to perform a ritual to reach across the boundaries of the world to call forth a companion based on your patron. Subtract an amount of mana equal to your mana limit each time you take a rest. Your eidolon is augmented to that amount of mana. See Companion for more details.
Your eidolon bears a glowing rune that is identical to a rune that appears on your forehead as long as your eidolon is summoned. While this rune can be hidden through mundane means, it cannot be concealed through magic that changes appearance, such as shapechange or polymorph (although invisibility does conceal it as long as the spell lasts).
As an action, you can expend 2 mana to replace one of your eidolon’s skill proficiencies with another skill proficiency.
If your eidolon is ever slain, the magical bond you share allows you to return it to life. With 8 hours of work and the expenditure of 25 sp worth of rare materials, you call forth your eidolon’s spirit and use your magic to create a new body for it. You can return an eidolon to life in this manner even if you do not possess any part of its body.
Your eidolon appears in a form based on your patron.
Aberrant. Your eidolon is an aberration, but uses a beast form. It has advantage on saving throws against spells and other magical effects.
Arctic. Your eidolon is an elemental: an ice elemental
Astral. Your eidolon is any companion form you choose. It no longer needs to breathe and it has advantage on saving throws to avoid being charmed or sent to another plane.
Beholder. Your eidolon is any companion form you choose.
Celestial. Your eidolon is a celestial, but uses a beast form. It gains resistance to radiant damage and has advantage on saving throws against being frightened.
Chaotic. Your eidolon is any companion form you choose.
Draconic. Your eidolon is a Dragon.
Fey. Your eidolon is one of the beast or fey forms.
Fiendish. Your eidolon is a fiend, but uses a beast form. Magical darkness doesn’t impede its darkvision. It gains resistance to fire and poison damage.
Genie. Your eidolon appears in an elemental form based on the type of genie your patron is. Dao: earth elemental, Djinni: air elemental, Efreeti: fire elemental, or Marid: water elemental.
Mummy. Your eidolon is an elemental: a sand elemental.
Phoenix. Your eidolon is a phoenix.
Sea. Your eidolon is an elemental: a water elemental.
Shadow. Your eidolon is an undead: a shadow, but uses a beast form. It has a shadowy form which allows it to enter a hostile creature’s space and stop there. It can also move through a space as narrow as 2.5 centimeters wide without squeezing and while in dim light or darkness, your eidolon can take the Hide action as a bonus action.
Spectral. Your eidolon is an undead, a ghost or specter, but uses a beast form. It can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.
Stone. Your eidolon is an elemental: an earth elemental
Storm. Your eidolon is an elemental: a lightning elemental.
Undying. Your eidolon is an undead, but uses a beast form. It is resistant to cold, poison, and necrotic damage and immune to the exhausted, frightened, and poisoned conditions.
Vampiric. Your eidolon is an undead, but uses a beast form. It has the Blood in the Water maneuver and it has resistance to necrotic damage.