A church warden and a magician befriended lonely pensioners before manipulating them to change their will, killing one and plotting to murder another, a court has heard.

Benjamin Field, 28, the son of a Baptist minister, has been accused of targeting older people for financial gain and coming up with “intricate” plots to kill his victims, including drugging, alcohol poisoning, suffocation, falls, attempts to cause heart failure, car crashes and unwitting overdoses.

He is on trial alongside 32-year-old Martyn Smith for the murder of 69-year-old Peter Farquhar in the village of Maids Moreton, Buckinghamshire, and has also been charged with the attempted murder of 83-year-old Ann Moore-Martin.

Oliver Saxby QC, the lead prosecutor, told a jury at Oxford crown court that Field’s motive was “financial gain”, adding that he had a “profound fascination in controlling and manipulating and humiliating and killing”.

He said Field, a church warden, suffocated Farquhar and tried to kill Moore-Martin but was foiled when her niece became involved. Moore-Martin later died of natural causes. Both deaths were meant to “look like accident or suicide”.

To carry out his “grand design”, Field had help from Smith, a magician, who was driven by greed and enthralled by Field’s plotting, the court heard.

For Field, “this was a project: befriend a vulnerable individual, get them to change their will and then make sure they died. And it is a project he seems to have relished devising and managing and executing and, to an extent, documenting – in various notes and diaries he made,” Saxby said in his opening statement.

“Indeed, piecing things together, it is clear that his project became his life’s work – a life’s work of which he was proud and for which he admired himself.”

Farquhar, a retired teacher who lectured part-time at the University of Buckingham, was described as an “intelligent and deeply Christian” man. Though he had friends, Farquhar was described as being “lonely in an emotional sense”.

Field and Smith met Farquhar when they were students at Buckingham. The defendants struck up a friendship with Farquhar and began lodging with him. “The first defendant [Field] saw that Peter was vulnerable. And this was something, from the very outset, he decided to exploit,” Saxby told the jury.

In an email entitled “An email about Peter”, Field described Farquhar as “a man of many contradictions. He is a closeted, Christian homosexual.” He referred to Farquhar’s desire for companionship. “He gives me things and he gets me for a length of time,” he wrote in the email.

Field and Farquhar entered into a relationship and had a formal ceremony, which they called a betrothal ceremony. In one diary entry, Farquhar described the event as: “One of the happiest moments of my life. Gone are the fears of dying alone.”

Moore-Martin, described by friends as “loving, kind and affectionate”, was someone for whom religion was very important, Saxby said. Moore-Martin lived a few doors down from Farquhar. Like Farquhar, she was unmarried and had no children. Saxby told the court she was “fundamentally lonely”.

Field and Smith embarked on a campaign of gaslighting to disorientate Moore-Martin and get her to question her sanity, the court heard. Both defendants wrote messages on Moore-Martin’s mirrors at home, which were biblical in nature, telling her to leave her house to Field. Moore-Martin did because she believed she was receiving messages from God. Both defendants also hid things around her house and encouraged her to thoughts of suicide, the jury was told.

Saxby QC quoted from one of Field’s books, which listed options for how to kill Moore-Martin, such as: “Heart attack – electrical device, dehydration, stair, sex?, in the bath? … OD on her prescriptions … church tower …sleep apnoea.” Their attempts to kill her culminated in a hospital admission, the court heard. While Moore-Martin was not subject to the same level of drugging as Farquhar, she did speak of Field giving her some white powder to make her sleep better.

Field and Smith are accused of targeting other local vulnerable elderly people in a series of burglaries – many of whose names were on a list prepared by Field under the heading “clients”, Saxby said. The pair are also accused of planning to deceive an elderly woman, now aged 101, whose will Field had on his university server and which Smith emailed to himself.

The third defendant, Tom Field, 24, the brother of the first defendant, “became involved on the margins”, the jury was told. He and his brother deceived Moore-Martin into giving £27,000 to buy a kidney dialysis machine on the false premise that the 24-year-old needed one to survive, Saxby said. The younger brother would pretend to be extremely ill when he and Moore-Martin met, “playing up to the idea he was in mortal danger and in dire need of help”.

Benjamin Field denies charges of murder, conspiracy to murder and attempted murder, and possession of an article for the use in fraud. He has admitted four charges of fraud and two of burglary.

He has admitted defrauding Farquhar and giving him drugs without his knowledge. He denies attempting to kill Farquhar and said he had drugged him because Farquhar had become difficult to live with. He admits defrauding Moore-Martin, but denies drugging her or trying to kill her.

Smith has been charged with the murder of Farquhar, several counts of fraud, conspiracy to murder Moore-Martin, and burglary. He denies any wrongdoing in relation to all charges

Tom Field has been charged with one count of fraud in relation to the dialysis machine, which he denies.

The trial will take place over the next 11 weeks.