Siralos

Siralos was a planet composed purely of psychic energy. It was said to be one of the 700 Wonders of the Universe.

History
The Master used the power of Siralos to catch the first seven incarnations of the Doctor in a series of traps. The Master also trapped several of the Doctor's enemies: Daleks, Sontarans, Cybermen, etc. in a number of dungeons and death traps.

The Doctor's servitor device, known as the Graak, was tasked with rescuing each of the seven Doctors from the Master's deathtraps. The Doctor's TARDIS as well as the Master's had both been tuned to Siralos so that the Graak could reach one of the Master's dungeons by crossing the Great Divide, a nexus which led either directly into the psychic energy of Siralos, or into one of its manifestations.

The Master assigned various missions to the Graak during his quest to rescue the Doctors. These usually took the form of riddles or retrieval of items. Upon sucessful completion of each mission, the opening of another portal across the Great Divide took the Graak to a more physical, action-oriented scenario. One such scenario had the Graak materialise within the Doctor's own brain. Another has the Graak engage in a space battle with a Dalek ship. Other scenarios included car and train chases, jousts with Sontarans and mazes. Upon the successful completion of these scenarios, the Doctor of that level was released from capture. (VG: Destiny of the Doctors)

Behind the Scene
Siralos was an anagram of Solaris, referring to the titular planet of a novel written by Stanislaw Lem, later adapted into two films. In the novel, Solaris causes illusions based on the thoughts or living beings close to it.