A Brooklyn man hacked out pieces of his girlfriend’s skull with a knife before running around naked and covered in blood while ranting about the devil, cops and witnesses said.

Miguel Cordero, 38, attacked Julissa Marquez, 32, before stalking the hallways and threatening others at the Sumner Houses in Bedford-Stuyvesant at 7:22 p.m. Tuesday night, according to law-enforcement sources.

“He was completely naked, blood from head to toe,” said a neighbor, Matthew Fuller. “He was screaming, ‘This is my building — I’m the devil’s child!’ ”

Another neighbor discovered the gruesome scene and the woman’s horrified 12-year-old son in the apartment.

“He said, ‘Help me, I’m scared. My mom is on the floor I think she’s dead,’ ” Louis Navarette said. “There was blood everywhere. There were pieces of her head missing.”

Cordero was arrested and taken to Woodhull Medical Center. Marquez was taken to Kings County Hospital, where, despite her wounds, she is expected to survive.