“You're not supposed to be in here! You're going to ruin the surprise!”The following section contains spoilers.
A Vitruvian is an Orokin recording device. One of these said devices plays a key role in The Sacrifice, containing recordings of Ballas detailing the creation of the Warframes, the Sentient's betrayal, and the Tenno.
While investigating Excalibur Umbra, the Operator and Ordis come across the Vitruvian in a hidden room on Lua. They discover that the Vitruvian contains Ballas's recordings, but much of it is locked. Installing the Vitruvian in the Orbiter, Ordis temporarily loses his "vestigial" precepts to "care" for the Operator, calling them "Star-Child", while displaying a new, emotionless personality. Virtuvian Ordis discovers a complete blueprint for Excalibur Umbra within the device.
The Operator builds Excalibur Umbra, only for him to display sentience. They discover that Umbra's memories contain the keys to unlocking the Vitruvian.
The Vitruvian contains lore regarding the Old War. After the Sentients were sent to terraform the Tau System only to gain sentience and were led by Hunhow to betray their creators, the Orokin cultured the Infested to be used as soldiers. However, when the Infested prove to be uncontrollable, the Orokin built Warframes, which are revealed to be Orokin soldiers infected by a unique strain cultured by the Helminth. However, the Warframes too proved to be uncontrollable until the Zariman survivors were used to inhabit the minds of the Warframes, hidden on Lua. It is then revealed that Ballas intended to send the Vitruvian to Hunhow and disclose the location of the Operators as part of a mutiny following the death of Margulis, but warns him to not underestimate the "devils" that were able to calm the Warframes.