The Void
Obeying none of the rules of normal space, time or causality; the shared ground of Tenno power and the Indifference threat.
LOHKFULL SPOILERS
Technological use
The Orokin used the Void through Solar Rails, the Reliquary Drive and other transit systems. Every one of those achievements carries the door Albrecht opened and a debt to a being no one understood.
Its effect on the living
It produced power in the Zariman children, madness in the adults, echoes after death in the Holdfasts and an entire country in Duviri. The effect is not uniform from target to target.
A limit of the canon
Whether The Man in the Wall is the Void itself or a separate being born within it is not settled. The Void and The Indifference should not be treated as synonyms.