The Codex Umbra is a character-exclusive item that Maxwell starts the game with. When used, it sacrifices 15 health and 55 of maximum sanity to spawn a Shadow Puppet clone of Maxwell. It has infinite durability and requires 2 Nightmare Fuel to be used. For each puppet active, Maxwell's maximum sanity is reduced, similar to how Meat Effigies reduce maximum health. A maximum of 3 puppets can be active at one time and each one will disappear 2.5 days after it was summoned. If a Shadow Puppet is killed or despawns, Maxwell will get the used max sanity back.
Origin
“I have discovered something, a book of sorts. I have yet to decode it fully, but what little I have deciphered has opened my mind to terrifying new possibilities.”
The Codex Umbra was discovered by William Carter in 1904, details are unknown.And Willam was intrigued by the "terrifying" possibilities it offered.[1] Months later, he established himself in San Francisco and began going by the name of Maxwell. Maxwell updated his act to incorporate dark manifestations that he taught himself to summon from the Codex Umbra.[2] He found great success in this new act and eventually hired an assistant, Charlie.[3] During their last performance, Maxwell's act went normally until he attempted to pull "shadows incarnate" from the Codex and was instead pulled into the book by giant shadow hands. Soon, the entire act ends in disaster as both Maxwell and Charlie are violently dragged into a parallel dimension.[4]
The Codex Umbra reappeared, still in Maxwell's possession, after he was dethroned and became a survivor. After reuniting with Wilson, Maxwell found the scientist's blueprints for an invention and offered to help construct it with assistance from the Codex Umbra. This made it possible for Maxwell and Wilson to build a portal that allowed other survivors to join forces with them.[5]
Trivia
Codex Umbra is Latin for "shadow manuscript".
It's possible for the Codex Umbra to be set alight and burn into ashes. The book will re-appear in Maxwell's inventory upon re-loading the world.
If using the Codex Umbra reduces Maxwell's health to zero, the Morgue will list the cause of death as "Shenanigans".
Like Abigail's Flower, the Codex Umbra floats when placed on the ground, but only if the corresponding Maxwell is nearby. If he moves out of range, the book will close and fall to the ground.
Bugs
Despite being a starting character specific item, the Codex Umbra can be stolen by mobs like Krampus or Eyeplants.
Using a Sinkhole to enter the Caves or Ruins without the Codex Umbra can sometimes cause it to duplicate in the player's inventory.
In the Shipwrecked DLC, using a Volcano to climb onto it without the Codex Umbra can sometimes cause it to duplicate in the player's inventory.
Gallery
Maxwell reading the Codex Umbra.
A puppet spawning (Look closer and you'll find Mr. Skitts).
Maxwell's maximum Sanity after summoning a Puppet.
Maxwell and the Codex Umbra on the All's Well That Maxwell poster.
Maxwell beside the Codex Umbra (DST).
Codex Umbra as seen on the 2017 Valentine Cards.
Codex Umbra animation concept from Rhymes with Play "Maxwell's Character Refresh"
References
↑William Carter Puzzle #4, The Postcard: "Jack, The strangest thing has happened! Please discount all reports of my demise, should they reach you. I am very much alive, despite my recent misadventures. I have discovered something, a book of sorts. I have yet to decode it fully, but what little I have deciphered has opened my mind to terrifying new possibilities. I shan't say more through post - I fear it may attract unwanted attention. All will be explained when we meet. I shall continue west forthwith! -William"
↑William Carter Puzzle #5: A new poster for "Maxwell the Great", a requisition for an ad in The San Francisco Call dated June 6, 1905, and a journal page recounting an encounter with a Shadow Creature after a ritual.
↑William Carter Puzzle #6: New poster for "The Amazing Maxwell" featuring Charlie holding the Codex, and a letter from Charlie about their latest successful performance.