The Obscura was a fracture in spacetime, which caused death if a person looked directly into it.
A Time Lord research station was set up around it to monitor it. Danna was stationed there by Irving Braxiatel. Many centuries later, Braxiatel returned with Ace to see if there was a way of using the Obscura's power as a weapon. Learning that there was a Dalek ship nearby, Braxiatel made them aware of their location with the promise of passing on Gallifreyan weapons to them. This was a lie and he opened the station's protective barriers, causing the Obscura's energies to destroy the Daleks.
However, the Daleks had exterminated Danna and Ace was almost killed by the Obscura's energies, so Braxiatel left with her, intending to wipe her memories of the Last Great Time War and Gallifrey. (AUDIO: Soldier Obscura)