Tristan Adams is the main antagonist of the Star Trek episode "Daggers of the Mind".
He was portrayed by the late James Gregory, who also played General Ursus in Beneath the Planet of the Apes and Bryagh in The Flight of Dragons.
Doctor Tristan Adams was the director of the Tantalus V penal colony, a facility for the criminally insane. He claimed to believe in humane prisons but his alternative was to develop a neural neutraliser, which allowed him to control brains and plant suggestions in them. He used the device not only to control and pacify the prisoners, but also the staff. His assistant, Simon Van Gelder, attempted to resist control but was driven almost insane. Despite this, when the Enterprise visited Tantalus V to collect some records, Van Gelder managed to hide himself in the consignment and was beamed aboard the Enterprise.
Although Van Gelder's ravings were initially dismissed, the discovery that he was a staffmember and not a prisoner prompted Captain Kirk to investigate the matter, beaming down to Tantalus V with psychologist Helen Noel. Adams gave them a tour of the facility and introduced them to the neutraliser in a casual manner. Kirk wanted to investigate further however and sneaked in with Noel to test the device himself.
Adams then discovered them and took charge of the situation, implanting the idea that Kirk loved Noel in his mind. He decided to use the machine on Kirk further but Noel escaped and shut down the colony's power supply. This allowed Kirk to break free and overpower Adams, leaving him lying in the neutraliser chamber. Spock beamed down to take control, shutting off the colony's shield and restoring power...which has the unintended effect of reactivating the neutraliser while Adams was lying under it. With no-one else present to control the effect on his mind, Adams died from the process.