The Arcane Codices, introduced in the Stolen Dreams, are quest items, each a highly valuable codex as described by multiple influential characters in-game. Together, they form a set of six, and all pieces are needed to access an Arcane Machine on an Orokin Derelict that the Grineer were attempting to access from Phobos.
Lore[]
Very little is known about the nature and content of the codices, however the Grineer were after them in hopes of finding a cure for Cloning Decay Syndrome and the Corpus in hopes of finding treasure. A thief named Maroo had been contracted to collect them for the Grineer. She later showed her interest in the potential of finding treasure with them during conversations with the Lotus.
Though the hunt for all six codices were eventually successful, what came out from the ancient machine was not what was expected. Cryptic and mysterious messages were broadcasted, after which a pair of Arcane Infested ambushes and attacks. After the encounter, the Lotus immediately concluded that "whatever the machine is doing, it's not for us".
The 'Womb of the Sky' mentioned in the Codices' transmission is the Tenno Reservoir, as confirmed by Hunhow.