A SCHIZOPHRENIC who claimed he was possessed by a mysterious force which pulled him into a frenzied attack on his elderly father was yesterday sent to a mental hospital for manslaughter. Mr Justice Bewley said it was clear that Cheng Po-kin needed psychiatric treatment and, in the public interest, he had to leave it to doctors to decide the day of his release. Cheng, 32, denied murdering his father, Cheng Sau-lin, 79, but admitted the lesser offence on the grounds of diminished responsibility. The court heard a second son, Cheng Po-wing, was in his room at the family's Pat Heung home on May 1 last year when he heard an ear-piercing scream. He rushed out and found his father crouching near the toilet, badly wounded and covered in blood, while his older brother was brandishing a chopper. The old man was found to have 16 chop wounds to his head and neck. His left hand had been completely severed and the right one was connected only by a narrow strip of skin. He died about five hours later in hospital. Cheng Po-kin told police he was possessed by a mysterious force which pulled him irresistibly from his chair into the kitchen, and that there, the chopper, which had eyes, hacked his father's head and hands. His counsel, Graeme Mackay, said although his client had been diagnosed as schizophrenic since 1982, there was no history of violence. He had probably been suffering from schizophrenic delusion and just ''snapped'' after years of criticism from his father.