Deus Codex's are MythicDrop Items which are used to 'identify' unidentified items. You can get Codex's rarely from mob drops, from daily and weekly chests, and by crafting if you are a level 30 Hunter. To use a Codex, simply right-click with it, and then right click with an unidentified item. This will consume the Deus Codex. Once identified, the item's tier and enchantments will be revealed.