surprise when his new form turns out to be prone to saying rude things. Regenerations can be traumatic. In Castrovalva, the Fifth Doctor requires the use of a Zero Room, a chamber shielded from the outside universe that provides an area of calm for him to recuperate. He comments that there is an excellent polygonal zero room beneath the junior senate block on Gallifrey. The Time Lord's personality also sometimes goes through a period of instability following a regeneration. It was first stated in The Deadly Assassin that a Time Lord can regenerate twelve times (thirteen incarnations in all). There were exceptions to this rule, however: when the Master reached the end of his regenerative cycle, he took possession of the body of another person to continue living. In The Five Doctors, the Master was offered a new cycle of regenerations by the High Council to save the Doctor from the Death Zone, which may indicate that there are methods to circumvent the twelve regeneration limit. The Master says in "The Sound of Drums" that the Time Lords "resurrected" him to fight in the Time War. It was revealed in The Brain of Morbius that the Time Lords also use the Elixir of Life in extreme cases, where regeneration is not possible. It is confirmed in "The Time of the Doctor" that a Time Lord can normally regenerate only twelve times but that the Time Lords have the ability to grant more regenerations: at the behest of Clara Oswald they granted the Doctor himself a new cycle when he was at the point of death from old age, having used up his entire cycle. Also in The Deadly Assassin, several Time Lords including the President are stated to have been "murdered" and are not stated to have regenerated. Regeneration, regardless of how many regenerations the individual Time Lord has already undergone, is a conditional and non-inevitable phenomenon. This is stated in "The End of Time" when the Tenth Doctor explains to Wilfred Mott that a Time Lord can die before they have a chance to regenerate, in which case they die outright. In The Deadly Assassin at least one of the murders was carried out with a 'staser', possibly a weapon designed to both kill and prevent regeneration (stasers are also stated to have little effect on non-living tissue). In the Series 4 episode "Turn Left", the Tenth Doctor's body