What Are The Codex:[]
It's important to know what "The Codex" are. Especially, since they are the most important thing in the game. You actually look for them throughout the game. So basically, "The Codex" are ancient manuscripts or texts in book form. As seen in the beginning of the game, when the cop rolls into the (Hell-fire).
Story:[]
The Codex is an ancient artifact that Deacon stole from the Grigori, containing "the Litany": an incantation that will open the Pit and release Asmodeus. This may only be performed when the Star Wormwood has arisen in the sky. It is therefore the Grigori's intention to reclaim the Codex that they may release their leader from his prison and bring about Armageddon. It is Deacon's plan to use this against them - by getting Shadow Man to open the Pit first and defeat Asmodeus.
The Codex is sealed with seven impenetrable metal straps of varying length into which fit the Sigils. As Shadow Man recovers Sigils throughout the game, these are placed into the Seals. As each of the Seals is filled with Sigils, it opens revealing another verse of the Litany, which when used in any of the Recreant Vaults within Gehenna allows access to the various Liveside levels.
(from Shadow Man 2econd Coming Game Scriptment)