Vashta Nerada

Last night I had a dream about Doctor Who, I won't get into the details but it gave me a really cool idea for a new Doctor Who episode. What if the Vashta Nerada of today are just poor copies of themselves. Way back in history they were destroyed, all but one. One Vashta Nerada (later the Doctor would realize there could be more than one) fell into a deep sleep. The Doctor would be faced with the choice. Kill it (next to impossible) or let it live? Because this to would be the "last" of an ancient race capable of destroying worlds. This creature would not only reside in a tree but actually is a tree. In slumber you would not be able to tell the Vashta Nerada tree apart from the others and since they are capable of sleeping for millenia even the Time Lords considered them nightmares to tell the young of Gallifrey. Like the boogie man.