The Pain Colony by Shanon Hunt

DEA Special Agent Peter Malloy is no stranger to the devastating impact of drugs. So when six bodies turn up with surgical ports implanted in their spines, he’s sure a potent new opioid is the culprit. But when lab tests show zero evidence of any known narcotics, he fears something far more sinister.

Allison Stevens thought she knew everything about her boss. But after he disappears, she’s stunned to discover his secret genetic research in pain tolerance. When she becomes the target of the FBI’s investigation, Allison has only one move. She must expose his illegal experiment. There’s just one problem. Some secrets will be protected at any cost.

Their search for answers leads them to a dark, eerie cult, where true believers seem perilously addicted to suffering, devoted to purification.