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For the quest item from Hexen, see Daemon Codex.
A codex entry for the Doom Slayer, taken from Doom (2016).

The Codex is the protagonist's mobile database of information on monsters, weapons, items, upgrades, and plot elements in Doom (2016) and Doom Eternal. It automatically updates 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.

In Doom (2016), some data log messages are acquired from datapads. In Doom Eternal and its expansions, almost every level contains flaming book pages that can be collected to unlock Codex entries; transmissions from the ARC can also be downloaded from the Fortress of Doom.