LIMA, Peru — Six months into the murder trial of Alberto Fujimori, prosecutors have produced little hard evidence that the former Peruvian president approved of a death squad to eliminate rebel collaborators. But they’re about to put a blockbuster witness on the stand in a trial that is riveting the nation.

The de-facto head of Peru’s intelligence service during Mr. Fujimori’s decade in power, Vladimiro Montesinos, allegedly organized the Colina group, a squad of army killers who slaughtered 25 civilians during Peru’s war against leftist rebels. He finally faces his former boss in court on Monday.

Mr. Montesinos, 63, was the shadow behind Mr. Fujimori as the two men crushed the rebels and cemented the autocratic leader’s popularity. He was accountable to none but the president, whom he preferred to meet in pre-dawn darkness. Secrets were his stock in trade. He paid off his opponents, or used information from his spy network to bend them to his will.

And by 2000, when Mr. Fujimori’s government collapsed in a corruption scandal involving Mr. Montesinos, many believe the spymaster’s power had grown to exceed even the president’s.

“Montesinos controlled the armed forces, the judicial system, the attorney general’s office. He had immense power,” a political scientist who reorganized Peru’s intelligence agency in a post-Fujimori government, Fernando Rospigliosi, said.

He has also been linked to some of the Fujimori government’s darkest days. A former death squad member testified he saw his leader meeting with Mr. Montesinos in 1991 a day after the squad, looking for a subversives’ gathering, showered the wrong barbecue party with bullets, killing 15 people including an 8-year-old boy.

Now prosecutors hope Montesinos will provide the testimony they need to convict his former boss.

Mr. Fujimori, 69, faces up to 30 years in prison and a fine of $33 million if found guilty.