Maleficent was an asteroid.
The Seventh Doctor knocked it out of the Kuiper Belt near the beginning of the 40th century so it would strike Uranus in 4010, resulting in a being of great wisdom and intelligence to be born millions of years later.
The collision was observed from the moon-turned-space station Cressida. Merculite tried to divert Maleficent to collide with the station instead, but the Doctor was able to return it to its original course. Prentis Duke stayed on Cressida to make sure the collision occurred, and was killed in the wake of the explosion. The Doctor travelled to the far future to witness the birth of the entity, which had some of Duke's identity as he had hoped. (PROSE: Uranus)