The ex-girlfriend of Seth Mazzaglia has testified that he killed Elizabeth "Lizzi" Marriott after placing a rope around her neck Oct. 9, 2012.
Kathryn McDonough, who struck a deal last year on conspiracy and witness tampering charges in the murder case, is testifying June 3 in Strafford County Superior Court in Dover, N.H.
She was the only other person there that night in October, and she later discarded evidence and helped Mazzaglia dump Marriott's body off Peirce Island in Portsmouth, according to court documents.
McDonough testified about Mazzaglia's "darkness," and his "Doomsday" threats, while he wanted her to spend more time with him.
"Who could keep the darkness at bay?" the prosecution asked.
"I could," she replied. "He said I was one of the few."
McDonough said Mazzaglia referred to themselves as "soul mates," and that she was "faking it" at times in regards to multiple "personas" she was reported to have had.
Her testimony is continuing, as of 10:30 a.m.