The laboratory was used for suspended animation research over 150 years before. Most of the researchers were killed by their hybrid creation, the Bloodhunter. Only Ravus Oldeman survived by putting himself in suspended animation. They revived Oldeman, but the Bloodhunter found and killed him. Oldeman's blood was used to reconstitute the Dark.
However, because Oldeman had become addicted to neurolectrin, the Dark also had that addiction. The Fifth Doctor injected the Bloodhunter with neurolectrin, causing the Dark to attack and feed on the hybrid. Stoker fired repeatedly at the Dark while it was occupied. When it was finished with the Bloodhunter, it snapped Stoker's neck. She had wounded the Dark enough, though, to allow the Doctor to use Stoker's lighter to help burn it to ash, destroying it. (PROSE: Fear of the Dark)