Watcher of Nineveh

The Watcher of Nineveh was a figure in Gallifreyan legend. It had lured many Time Lords to their deaths. It resided on a place called Nineveh, which the Doctor described as a kind of "null space".

During the Doctor's seventh incarnation, his TARDIS brought him, against programming, to the null space. The Doctor investigated, only to discover it was a junkyard full of old TARDISes. His exploration was cut short, however, by the arrival of the Watcher of Nineveh.

The Doctor initially tried to fight back against the Watcher, but quickly discovered the being had no corporeal existence. Seeing no alternative, the Doctor simply ran, eventually finding his way back to the TARDIS. Even this was no sanctuary, however, since the Watcher could penetrate and inhabit the TARDIS.

In the end, the only thing that saved the Doctor was the Watcher's recognition that the Doctor was in his seventh body. Since Time Lords had thirteen lives, it was not yet time for the Doctor to die. (COMIC: Nineveh!)

Behind the scenes
After the airing of the episode, The Doctor's Wife, it was noted that the two stories had a lot in common. The Watcher of Nineveh's counterpart is House, a similar incorporeal entity which murdered many Time Lords and was able to penetrate TARDISes, resulting in a TARDIS junkyard.