From Destinypedia, the Destiny wiki
Tartarus is an Vex domain located deep within the The Nexus. The terminus domain was used by the Vex to eradicate old codes and abandoned terraforming projects. The architect of the Endless Night rerouted directives into the domain to maintain the simulation against the Last City.
The domain is governed by the Oppressive Mind, Dimio and nearly collapsed following the defeat of the Mind. However, a Taken incursion infected the domain and prevented its destruction. The Oppressive Mind was reborn with corruption by the weaver, Quria, the Dreaming Mind.