The Spriggan

 was an alternate version of the Tenth Doctor. He became a terror to his planet and promised to take Jessica Kelly on her 18th birthday and killed Kathy Kelly due to Peter Kelly's actions to stop him arriving at Jessica's birth.

He encountered the Doctor when he came to claim Jessica and killed Peter for his defiance. The Spriggan was using the youth of a village to power his TARDIS. The Doctor thought that he came from another dimension which was more dreamlike. He was getting the remnants of the Spriggan's TARDIS' energy back from the planet that left it when it crashed. He created Leela, but she rebelled against him to protect the Doctor, throwing him into the Vortex. (AUDIO: Splinters)