Army machine-gunner Caleb Daniels lost his best friend and seven other members of his unit when a Chinook helicopter — one he was meant to be on — crashed in Afghanistan.

The 2005 tragedy haunted him when he returned to his home in Savannah, Ga. At night, a tall, shadowy figure crept into his room. Sometimes the Black Thing would threaten to kill him; other times it would choke his dead best friend.

The dark figure, a “Destroyer demon,” punished him, he said, “for killing and for living.”

Without answers — his PTSD diagnosis offered little explanation — he went to the one person he felt could save him: a minister who offered $199 exorcisms out of his trailer.

Daniels, profiled in the book “Demon Camp” by first-time author Jennifer Percy, is just one of many deeply troubled soldiers suffering from the after-effects of war who are so desperate for respite they undergo exorcisms at a fringe Pentecostal retreat.

Bear Creek Ranch, in Portal, Ga., is ministered by Tim and Katie Mather, a husband-and-wife team that has conducted over 5,000 exorcisms, some of them on veterans.

This is perhaps not surprising given the numbers of the afflicted.

Of the 2.6 million veterans who served in Iraq and Afghanistan, an estimated 20% fit the criteria for post-traumatic stress disorder, where a person continues to relive a trauma, causing nightmares, flashbacks, negative feelings and hyper-arousal (a feeling of being “keyed up” as in battle). Psychiatric symptoms — like Daniels’ hallucinations and delusions — are not included in the classic definition of PTSD symptoms but have been reported in as high as 64% of sufferers.

Some researchers believe that PTSD with psychiatric symptoms deserves its own diagnosis. Others cite evidence that PTSD sufferers with psychosis often have underlying mood disorders that preceded the trauma of war.

Either way, the visions that haunted Daniels’ nights became untenable. He woke his wife with shrieks and cries. His dead buddies crowded his bedroom. “Everywhere he went, he saw them, their burned bodies, watching him,” writes Percy.

That’s when the Destroyer, a 6-foot-5-inch buffalo with horns, entered his life — and refused to leave. “It was a shadow. It was death. It was the gathered souls of all of his dead friends,” she writes.

The demon-battle continued until a fellow veteran suggested he visit Bear Creek Ranch.

“Society thinks PTSD cannot be healed, but society is wrong,” the fellow warrior told Daniels. “It’s called deliverance. It works wonders.”

According to the website, the retreat’s philosophy is “based in Luke 4:18, declaring the good news of the Kingdom, healing for the broken hearted, freedom for the captives, liberty for the bruised and recovery of spiritual sight.”

The Mathers believe, in Percy’s words, that “people are in bondage to a pattern of sin. Trauma is the doorway through which demons can pass.”

The three-day retreat includes food, housing, a demon-fighting workbook authored by Tim Mather and a 30-minute exorcism.

Each retreat is capped at 15 people (and is often sold out, they add). The exorcisms are done wherever there is space. Sometimes in a local church, abandoned buildings, or even in their trailer. They begin with worship. Then they “secure the perimeter” with the help of “intercessors” or people scattered across the country praying for God’s help.

Then they bring in the possessed — in this case, Daniels, a special-operations sergeant who survived his own helicopter crashing in a Taliban stronghold in the middle of the Afghan desert. (He was so convinced of his looming death that he ate his most prized possession: a photograph of his newborn daughter.)

A group of people sit around him in foldout chairs, sharing visions. One saw Daniels’ wrist bound with barbed wire; another saw him tied to the railroad tracks as a train approached. The minister saw a “worn and tired” angel fighting with his Destroyer demon.

As the group surrounded Daniels and prayed, he “felt a burning sore rip open on the back of his neck. It felt as if the flesh was coming off and something was being pulled up his spine toward the burning.”

Daniels tells Percy he felt the “hot Jesus blood coming down over his face.” Then “a glowing thing moved down his legs.” The minister reached out his hands and announced: “Caleb, you have a reason to live.”

Daniels became an overnight convert, spending his time gathering up damaged soldiers and bringing them to the exorcism camp. With his help, Mather has met over 400 demons and has bought “thousands” through deliverance, he tells Percy.

Percy wonders if this is some kind of “exposure therapy.”

In those with PTSD, “exposure therapy” in the form of war-simulation video games, like “Virtual Iraq,” to help the fractured mind “assimilate and process the event.”

“I wonder if Caleb has invented his own ‘Virtual Iraq,’ his own traumatic repetition. Every time he sees a demon, he fights it. And like the controlled redemption of ‘Virtual Iraq,’ patients cannot die or suffer wounds; Caleb always wins,” she writes.

But whatever good might be attributed to these exorcisms, Daniels, now in his early 30s, has since cut ties with the demon camp, he tells Percy. “They messed up my friends . . . they get in your head,” he said.

How own demons haven’t been fully exorcised. By the end of the book, Daniels continues to speak of them.

“You think Afghanistan is scary? You think a f – – – ing IED is scary? Rockets? Dead guys everywhere? It’s nothing compared to this war. This war is much, much worse,” he told Percy.

Bear Creek Ranch is still up and running — when reached for comment, the Mathers said they were too busy with a weekend retreat for an interview.

But they did have some words for Percy, even though they had not yet read her book. “Much of what she’s written is creative embellishment, and I’m sorry that the book has been published.”