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A codex entry for the Doom Slayer.
For the quest item from Hexen, see Daemon Codex.

The codex is the protagonist's mobile database of information on monsters, weapons, items, upgrades, and plot elements in the 2016 Doom. It is known to automatically update with new information as monsters are killed and items are collected, and can be referenced at the player's convenience, taking the place of Doom 3's PDAs and functioning in a manner similar to the scan log in the Metroid Prime series.