The ties that bind the "surprise witness", Mr Edgar Matobato, and the lead investigator, Senator Leila de Lima, date back to 2009.
Back then, Mr Matobato was part of a scuttled plot to kill Ms de Lima. Now he is helping her make a case against extrajudicial killings that have blighted President Rodrigo Duterte's anti-crime war.
Mr Matobato was a member of an anti-communist vigilante group in Davao and only in his late 20s when he was drafted in 1988 into a band of ruthless killers known in Davao by the seemingly whimsical moniker, "Lambada boys".
Now he is 57, already with white hair, but speaks with a steady voice and has the thick fingers of a man used to hard labour.
Mr Duterte's allies have already begun to poke holes into his story. He was told, for instance, that he named the wrong location where a businessman was killed supposedly over a feud with Mr Duterte's son.
Yet, the most common rebuttal has been that his testimony consists of rehashed allegations and urban myths about Mr Duterte that are being dredged up by his enemies.
The ringleader of this plot to unseat him, they say, is Ms de Lima. Back in 2009, as human rights commission chief, she opened investigations into Davao's "death squads".
In 2013, the commission linked Mr Duterte to these groups and recommended that charges be filed against him. But an anti-graft court said last year it did not find enough evidence to indict Mr Duterte. In January this year, as Mr Duterte was running for president, the court closed the case when it lost the only witness it had willing to testify to the killings.
That witness was Mr Matobato. He had asked to leave his death squad in 2013. "I was bothered by my conscience, and I was already getting too old," he said.
But he was implicated in the killing of a wealthy hotelier, Mr Richard King, in 2014, and tortured.
Fearing for his life, he ran to Ms de Lima, who was then justice minister. She placed him under witness protection last year. When it became apparent Mr Duterte would become president, Mr Matobato left the programme in January. He then turned to a priest who advised him to tell the Senate everything.
That landed him under the protection of Ms de Lima, who as senator is again going after Mr Duterte.
She insisted that she has no agenda other than exposing the truth: "I feel the people should know."
Mr Matobato will again testify before the Senate next Tuesday.