A church warden and a magician "spiked" a retired lecturer's food and drinks during a sustained plot to murder him, a court heard.
Benjamin Field, 28, and Martyn Smith, 32, are accused of planning the deaths of Peter Farquhar, 69, and Ann Moore-Martin, 83, in Buckinghamshire.
Oxford Crown Court heard Mr Farquhar's whisky was "supercharged" and his food laced with hallucinogenic drugs.
The accused deny charges of murder and conspiracy to murder.
The court heard the accused "psychologically manipulated" the deeply religious pair who lived three doors from each other in the village of Maids Moreton in Buckinghamshire.
Oliver Saxby QC, prosecuting, said the plot was a sustained campaign of so-called "gaslighting" - making them believe they were losing their sanity.
Mr Farquhar, who taught part-time at the University of Buckingham, suffered night terrors and hallucinations and recorded this in a handwritten journal, Mr Saxby said.
The barrister told the court the 69-year-old's drinks were topped up with bioethanol and poteen, a high strength Irish alcohol, and his food was laced with drugs.
"Peter Farquhar underwent various humiliations culminating in a doctor advising that all alcohol should be removed from his house and that he should abstain from drinking," the court was told.
"Peter Farquhar was not an alcoholic. Nor was he suffering from dementia, nor did he have any other form of disease of the mind."
Mr Saxby alleged that after Mr Farquhar changed his will three times in two years to benefit Field and Mr Smith, he "had to die".
Mr Farquhar died in October 2015, while Miss Moore-Martin, a retired teacher, died of natural causes in May 2017.
Field, of Wellingborough Road, Olney, denies murder, conspiracy to murder, possessing an article for the use in fraud and an alternative charge of attempted murder. He has admitted four charges of fraud and two of burglary.
His brother Tom Field, 24, of Wellingborough Road, Olney, Buckinghamshire, denied a single charge of fraud.
Mr Smith, of Penhalvean, Redruth, Cornwall, denies murder, conspiracy to murder, two charges of fraud and one of burglary.
The trial continues.