Derek Clark, 32, attacked grandmother Mary Coulter after he and two accomplices went to her home in search of her son Ronnie.
He was looking for Mr Coulter amid claims he had terrorised Clark's family.
Judge Lord Stewart, at the High Court in Glasgow yesterday, said photographs of Mrs Coulter's injuries "should come with a health warning" .
Clark was in court after admitting to the attempted murder of the 76-year-old.
Prosecutor Alex Prentice, QC, said Clark was high on cocaine as he left his home in Ashgill, South Lanarkshire before the attack in the early hours of March 22 this year.
He was joined by friends Richard Parvin, 29, and 27-year-old Darryl Finnie. All were wearing balaclavas and Clark was armed with a large knife. The court heard how Mrs Coulter was in bed at home in Overtown, North Lanarkshire, when she heard banging at her front door. She went downstairs and could see the trio looking into rooms in her house.
Mr Prentice said: "Clark – without provocation – hit her on the right said of her face just below the eyes. She immediately felt the impact, put her hands to her face and felt wet blood."
A neighbour heard Mrs Coulter shouting for help once the trio had left. The court heard that the men were in Finnie's mother's home in Carluke, South Lanarkshire later that morning. She asked why they were there and Clark told her: "You'll know soon enough – it will be all over the news."
Mrs Coulter required an emergency blood transfusion and facial reconstructive surgery. Her injuries included a 15cm wound across her face and a 10cm gash to her head resulting in a skull fracture.
Parvin and Finnie pled guilty to conspiring to inflict injury on Ronnie Coulter.
Clark's QC said he had gone looking for Ronnie Coulter that night and that he "bitterly regrets" what happened to the pensioner.
Ian Duguid, defending, added: "He maintains that he did not know he was striking Mrs Coulter."
Lord Stewart deferred sentencing until later this month.