A woman is on trial for murder after allegedly incapacitating a businesswoman with a stun gun before she was stabbed to death.
Sarah Williams, 35, was described in Preston Crown Court as a jealous and obsessive woman who planned and carried out the killing of someone she regarded as a love rival.
Mr Johnson, with Ms Hartley in 2013, was abroad at the time of the murder
Williams is accused of killing "decent, hard-working" Sadie Hartley, 60, by stabbing and slashing the mother of two 40 times in an "orgy of violence" on the doorstep of her home in the village of Helmshore, Lancashire.
The defendant had previously been in a relationship with Ian Johnston, 57, Ms Hartley's partner, but he ended the romance because he felt Williams was too "possessive and difficult".
Prosecutor John McDermott told the court how "obsessed" Williams "set her mind" to getting her past love back but saw Ms Hartley as the "obstacle" to her desire.
The court heard the killing took place in the evening of 14 January when there was a knock on the victim's door.
Ms Hartley lived in the Lancashire village of Helmshore
Mr McDermott said: "What happened next is truly shocking.
"Sarah Williams stood on the doorstep. As soon as the door was opened we suggest he lunged at Sadie Hartley with of all things a stun gun - the sort of thing you might use legitimately to prod cattle.
"She pressed it against her - Sadie Hartley's head - and incapacitated her.
"Then with what can only be described as almost demonic savagery, she attacked her with a knife.
"She stabbed and slashed at this unfortunate woman; blow after blow, causing appalling and fatal injuries.
"She left her victim in a pool of blood in the hallway; closed the door; walked back to the car she had used on her murderous mission and set off back to her home in Cheshire."
Williams, who denies the charge, is accused of recruiting a second defendant, Katrina Walsh, 56, also from Chester, to help her with the "murderous mission".
The court was told how Walsh, who denies murder, kept a "revealing" diary detailing the plot in which she said she was "unexpectedly excited" by the plan and wrote about the victim as a "totally evil" woman.