Crozier is a major antagonist in the "Mindwarp" segment of the Doctor Who story "The Trial of a Time Lord".
He was portrayed by Patrick Ryecart.
Crozier was a human scientist employed by the Mentors, who wanted to make use of his talents in genetic engineering. As an early test of his skills, he augmented a non-sentient sea monster, the Raak, to operate complex equipment. His main task was to find a way to prolong the life of Lord Kiv, whose brain was expanding to the point where it would be crushed inside his skull. However, he was also used to attempt to brainwash the Krontep king Yrcanos.
When the Doctor stumbled upon Crozier's operations, Sil had Crozier use his mindwarp machine on him. This caused the Doctor to behave in an unpleasant and selfish manner, switching allegiance to the Mentors and helping Crozier repair his equipment. Crozier managed a temporary respite by transferring Kiv's mind into the body of another Mentor, but needed a humanoid body and selected that of Peri.
According to the version of events presented by the Valeyard at the Doctor's trial, Crozier succeeded in transferring the contents of Kiv's mind to Peri's brain, killing Peri in the process. The Time Lords, worried about the fact Crozier had effectively found a way to make people immortal, arranged for Yrcanos to attack the laboratory in the aftermath, apparently killing everyone present.
As was later revealed, Peri had survived. It is generally assumed that Yrcanos attacked the laboratory and killed Crozier before he could carry out the procedure. However, the Big Finish audio play "Antidote to Oblivion", which featured Crozier's daughter, stated that he was killed by Sil.