The Determinant

The Determinant was a realm created and controlled by the Master from the planet of Siralos, which was made of pure psychic energy. The Master planned to use it to bend the universe to his will and had captured seven of the Doctor's incarnations. He was defeated by the Graak, a creation of the The Doctor, who completed the various challenges in Determinent set up by the Master. (VG: Destiny of the Doctors)