A mother killed her partner and their four-year-old daughter to "prevent the world being taken over by vampires", a court heard.
Shelley Christopher, 36, was mentally ill when she stabbed 42-year-old Richard Brown, 29 times and daughter Sophia six times before inserting wooden objects into their bodies to stop them turning into the fictional demons.
Christopher also attacked another child and put a pencil in her body, but despite her injuries, the girl, who cannot be identified, survived, prosecutor Crispin Aylett QC told jurors.
Two days before the killings in February, Christopher, of Colville Square, Notting Hill, west London, went to a Mental Health Unit in north Kensington where she told staff that someone was "out to get" her.
But she refused pleas by a doctor and nurse to stay at the unit and went home.
Christopher is on trial at the Old Bailey on two counts of murder and one attempted murder which she denies by reason of insanity.
Opening the trial, Mr Aylett told jurors: "I'm afraid that this is a distressing case which you will find both terrible and tragic.
"Ms Christopher was later to tell a psychiatrist that, on the day of the killings, she had received a signal instructing her to kill her family in order to prevent the world from being taken over by vampires."
"The signal had come from a light bulb in the ceiling. She had done - or tried to do - what she was told.
"After she had attacked each of them with a knife, the light bulb had told her to put something wooden into each of their chests in order to stop them from becoming vampires."
The prosecutor went on: "That Ms Christopher must have been mentally unwell at this time is borne out by the findings of the doctors who examined the victims.
"From Richard's chest cavity, the pathologist recovered part of a child's paint brush. The pathologist who examined Sophia's body retrieved part of a pencil."
A psychiatrist concluded that Christopher, who is now in a secure mental hospital, had been suffering from a psychotic illness - most probably paranoid schizophrenia - when she killed her partner and child.
Mr Aylett told jurors that when a defendant enters a plea of not guilty by reason of insanity, it is for them, not a judge or psychiatrist, to decide the case on the evidence.