Reliquary Drive
The travel mechanism at the heart of the Railjack and Void transit, tied to the finger severed from The Man in the Wall.
VOID TRAVELFULL SPOILERS
The first finger
As Albrecht fled the portal, the reaching being's finger was caught in the closing door. Studying that remnant, the Orokin turned it into the physical constant of Void technology.
Use aboard a Railjack
Cy's ship crosses between Proxima regions and Void Storms. The crew's interstellar movement rests on the remains of a being no one understands.
An open question
Whether every Reliquary holds that same original finger or a derived copy has never been fully explained. What is certain is that human travel depends on the body of The Indifference.