Gareth John Womersley was part of a group that had travelled to an address in Castleford after his distressed son rang him and said he had been attacked at a party.
Prosecutor Louise Pryke told Leeds Crown Court that the 38-year-old got in a car with others and drove to Kendal Drive in the early hours of February 27.
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It is thought that Womersley's son had been involved in a violent altercation with another male, whose grandparents were already on the street.
Womersley is reported to have got out of the Mini with his group and began shouting "who has assaulted my son?" while swinging the Samurai sword around. He then put the sword towards the grandmother's throat.
The grandmother had her hair pulled by one of Womersley's group, while the grandfather said he was punched by another.
The police turned up and the group drove off.
Womersley, of Keswick Drive, Castleford, was later arrested and questioned, before being held on remand until sentencing this week.
Appearing via video link from HMP Leeds, he admitted a charge of affray and having an offensive weapon in public.
The court was told that he has eight previous convictions for 19 offences, including multiple actual bodily harm convictions from 2003 and 2005.
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Mitigating, Satpal Roth-Sharma said Womersley admitted he was "slightly intoxicated" that evening when his son phoned him.
She said: "Panic set in, and he got in the vehicle with others and attended the scene. He acted, on reflection, in a way he ought not to have done. He does show remorse and regret.
"He thought his son was in danger and acted like any parent would, minus brandishing a weapon in the street."
Judge Mustaq Khokhar jailed Womerlsey for nine months, saying: "Imagine if the other people had been bloody-minded and carried weapons, what would have happened?
"People have to be deterred from pursuing this course of action."
This article was first published by the Wakefield Express.