A student has been sentenced to life in prison for the murder of his step-grandmother after confessing to the crime in a game of truth or dare. Tiernan Darnton, 21, who was caught after a counsellor alerted police to the confession, was told by a judge at Preston crown court that he must serve a minimum of 15 years of his sentence. His 94-year-old step-grandmother, Mary Gregory, was discovered under a table in the conservatory of her smoke-filled bungalow in Heysham, Lancashire, in May 2018. She died four days later in hospital. Her death was initially treated by fire investigators as an accident caused by a cigarette. But police reopened the case in May 2019 after being alerted to what Darnton had told a counsellor. He said he had killed Gregory by setting fire to a curtain at her home and that others knew about the crime. Darnton had made a similar confession several weeks after Gregory’s death, during a game of truth or dare with two friends, the court heard. “I have a secret I haven’t told anyone. I may have killed someone,” he told the pair. When pressed, he said he started the fire because he did not want Gregory to suffer any longer from dementia, the court heard. He told the counsellor in May 2019 about a friend “who could send me to prison cos of what he knows”. A week later he told the counsellor and his stepfather, Chris Gregory, 66, that he had started the fire, the court was told. The counsellor then informed the police. Sentencing Darnton to life, Mrs Justice Yip told him: “Murder had been on your mind for some time. On your own evidence, you were fascinated by serial killers and their crimes. You had dark thoughts. Internet searches which you made before and after you killed Mrs Gregory paint a worrying picture. “Your opportunity to act on your dark thoughts came when your stepfather was away. In the early hours of 28 May 2018, you went to Mrs Gregory’s home and deliberately started a fire in one of the bedrooms.” The judge went on: “It is hard to imagine the horror Mrs Gregory must have felt when she realised her house was on fire and was filling with smoke. Neighbours heard her screams. Despite her frailty, she tried to get out, but she had been trapped. The fire service found her near to the conservatory doors, where you had blocked her exit. “Her final days were spent in hospital. Quite understandably, her condition in those days continues to haunt those who loved her. The manner of her death was particularly cruel.” The court heard that an examination of his mobile phone and laptop found internet searches made after the fire including “I’m a murderer”, “I’m a monster and I’m going to hell”, “I want to cause evil” and “urge to kill again”.