Ambition Parasites were parasites which fed on the potential and motivation of other life-forms.
The ambition parasites had no physical form and were made of psychic energy. When an ambition parasite arrived in an area, it needed to latch onto the oldest living organism in the area to survive. It would psychically drain the motivation out of everyone around it. In emergencies, it would attack with psychic energy and would force everyone it could control to defend it. The only ones who were resistant were those with a strong mind and those who had no goals, like children. (PROSE: Number 1, Gallows Gate Road)
At some point before the 18th century, an ambition parasite came from Esto to Earth and infested an oak tree in what would be later known as Gallows Gate Road. In the 18th century, it was used for hanging criminals, giving the road its name. It developed a reputation as being cursed, due to the predation of the ambition parasite.
In 1940, the Tenth Doctor's TARDIS detected the Ambition Parasite in the area. It dropped the Doctor off at the house on the road, but didn't inform him of what was needed. Eventually, the Doctor was able to work out what he had to do, though the parasite attacked him psychically when he attempted to return to the TARDIS. With the help of Robert Mann, he was able to find out where it was. He then confronted the parasite, and when it fought back, Robert started cutting down the tree. As the parasite weakened, the other tenants at the boarding house joined in, until the host was destroyed and their minds were freed.