A pensioner believed messages written on mirrors telling her to change her will and leave her home to her young lover came from God, a court heard.
Benjamin Field, 28, left Biblical messages for 83-year-old Ann Moore-Martin at her Buckinghamshire home.
Mr Field is accused with his friend Martyn Smith, 32, of murdering Ms Moore-Martin's neighbour and of planning to kill her.
Both he and magician Mr Smith deny the charges at Oxford Crown Court.
Mr Field, a Baptist minister's son, sent letters and postcards to the retired headmistress as part of a year-long seduction.
The church warden also wrote messages urging her to give him £27,000 towards the cost of a dialysis machine for his seriously ill brother, Tom, 24.
Miss Moore-Martin, a deeply religious retired teacher who never married or had children, later changed her will to leave her Maids Moreton home to Mr Field.
But according to the court she changed her mind and accused Field of poisoning her shortly before she died in May 2017.
'He will return'
The jury was told Field had taken photographs of the messages which were later recovered from his phone.
One message, illustrated with a cross, said: "All that you give him, He will return, Ten fold."
Mr Field had been introduced to her by her neighbour, university lecturer Peter Farquhar, 69, who Ben Field was living with and planned to marry.
Prosecutors said Mr Field targeted Miss Moore-Martin a few months after allegedly murdering Mr Farquhar.
Ben Field and Mr Smith deny murder, conspiracy to murder and possession of an article for use in fraud.
Mr Field, of Wellingborough Road, Olney, Buckinghamshire, also denies an alternative charge of attempted murder.
He has admitted four charges of fraud and two of burglary.
Mr Smith, of Penhalvean, Redruth, Cornwall, also denies two charges of fraud and one of burglary.
Tom Field, also of Wellingborough Road, Olney, denies a single charge of fraud.
The trial continues.
Correction 21 May 2019: An earlier version of this story listed incorrect charges against Tom Field. These have now been amended.